sysroot: fix missing symlinks for blas and lapack [IO-60] (#25)

To support cross-compiling from x86_64 to aarch64 debian targets - we're
using some scripts from the chromium
project for building sysroots directly from debian (`.deb`) packages.

A caveat of using these scripts is they rely on very low level plumbing
commands (i.e. `dpkg-deb`) to handle unpacking
and installing the package contents (libs and headers), into "jail" (the
sysroot).

These commands don't handle many details of the full install process. In
the case of blas and lapack - they don't handle the post install step of
creating symlinks at `/usr/lib/${TRIPLE}/libblas.so`. This is important
because this location is on the linkers default search path. The reason
these symlinks are not part of the package by default is because blas
and lapack are virtual packages, there are several different "concrete"
packages that can satisfy the dependency (i.e. `libblas-dev`,
`libopenblas-dev`, etc..). It's up the the debian alternatives system (&
system administrators) to handle the detail of which concrete library
the `libblas` and `liblapack` symlinks actually point to.

I explored a couple of routes to try to do this in a less janky way then
manually creating the links, but this ended up being a rabbit hole.
Ideally we revisit the way we're building these sysroots in a more
robust way.

In the meantime we can now remove the hardcoded linker paths in
libraries that depend on `blas` and `lapack`:

https://github.com/swift-nav/rules_swiftnav/blob/d10814e9b8f3d3b948e3ea9c1aed9c80c2fd74cc/third_party/suitesparse.BUILD#L25.
Trying to handle all the different conditions (os, arch, sysroot or no
sysroot, etc..) was becoming untenable, and just didn't work at all in
some cases.

## Testing

I've tested the resulting sysroot build locally to verify that this
mechanism of creating the links works.

sysroot/sysroot-creator.sh

@@ -361,45 +361,29 @@ StripChecksumsFromPackageList() {
 HacksAndPatches() {
   Banner "Misc Hacks & Patches"
 
-  # Remove an unnecessary dependency on qtchooser.
-  # rm "${INSTALL_ROOT}/usr/lib/${TRIPLE}/qt-default/qtchooser/default.conf"
-
-  # libxcomposite1 is missing a symbols file.
-  # cp "${SCRIPT_DIR}/libxcomposite1-symbols" \
-  #   "${INSTALL_ROOT}/debian/libxcomposite1/DEBIAN/symbols"
-
-  # __GLIBC_MINOR__ is used as a feature test macro.  Replace it with the
-  # earliest supported version of glibc (2.26, obtained from the oldest glibc
-  # version in //chrome/installer/linux/debian/dist_packag_versions.json and
-  # //chrome/installer/linux/rpm/dist_package_provides.json).
-  local usr_include="${INSTALL_ROOT}/usr/include"
-  # local features_h="${usr_include}/features.h"
-  # sed -i 's|\(#define\s\+__GLIBC_MINOR__\)|\1 26 //|' "${features_h}"
-
-  # fcntl64() was introduced in glibc 2.28.  Make sure to use fcntl() instead.
-  # local fcntl_h="${INSTALL_ROOT}/usr/include/fcntl.h"
-  # sed -i '{N; s/#ifndef __USE_FILE_OFFSET64\(\nextern int fcntl\)/#if 1\1/}' \
-  #     "${fcntl_h}"
-
-  # Do not use pthread_cond_clockwait as it was introduced in glibc 2.30.
-  local cppconfig_h="${usr_include}/${TRIPLE}/c++/10/bits/c++config.h"
-  sed -i 's|\(#define\s\+_GLIBCXX_USE_PTHREAD_COND_CLOCKWAIT\)|// \1|' \
-    "${cppconfig_h}"
-
-  # This is for chrome's ./build/linux/pkg-config-wrapper
-  # which overwrites PKG_CONFIG_LIBDIR internally
-  SubBanner "Move pkgconfig scripts"
-  mkdir -p ${INSTALL_ROOT}/usr/lib/pkgconfig
-  mv ${INSTALL_ROOT}/usr/lib/${TRIPLE}/pkgconfig/* \
-      ${INSTALL_ROOT}/usr/lib/pkgconfig
-
-  # Avoid requiring unsupported glibc versions.
-  "${SCRIPT_DIR}/reversion_glibc.py" \
-    "${INSTALL_ROOT}/lib/${TRIPLE}/libc.so.6"
-  "${SCRIPT_DIR}/reversion_glibc.py" \
-    "${INSTALL_ROOT}/lib/${TRIPLE}/libm.so.6"
-  # "${SCRIPT_DIR}/reversion_glibc.py" \
-  #   "${INSTALL_ROOT}/lib/${TRIPLE}/libcrypt.so.1"
+  # In debian blas and lapack are virtual packages.
+  #
+  # As such - the alternatives system is responsible for ensuring that a
+  # libblas.so and liblapack.so are available in /usr/lib/${TRIPLE} -
+  # i.e. - they are in the linkers default search path.
+  #
+  # The implementation we're using here - libblas-dev, and liblapack-dev
+  # only install the libraries to /usr/lib/${TRIPLE}/blas and /usr/lib/${TRIPLE}/lapack
+  # which are not on the linkers default search paths.
+  #
+  # Typically the symlink creation is handled by their respective post install
+  # scripts - but since we're using dpkg-deb directly - we need to do it ourselves.
+  #
+  # Otherwise we have to hardcode the install locations in the build system.
+  # Dealing with the possible combinations of arch, os, whether we're building
+  # with a sysroot or not, etc... is too much to keep straight. So we'll deal
+  # with it here.
+  cd ${INSTALL_ROOT}/usr/lib/${TRIPLE}
+  ln -s ./blas/libblas.so libblas.so
+  ln -s ./blas/libblas.a libblas.a
+  ln -s ./lapack/liblapack.so liblapack.so
+  ln -s ./lapack/liblapack.a liblapack.a
+  cd -
 }
 
 InstallIntoSysroot() {
@@ -528,7 +512,7 @@ BuildSysroot() {
   local files_and_sha256sums="$(cat ${package_file})"
   StripChecksumsFromPackageList "$package_file"
   InstallIntoSysroot ${files_and_sha256sums}
-  # HacksAndPatches
+  HacksAndPatches
   # CleanupJailSymlinks
   # VerifyLibraryDeps
   CreateTarBall