Conform builds to manylinux2014 glibc standards

[bjia56]

Restructures the Linux build to do the compiling inside Centos 7 Docker containers published by the manylinux team for x86_64 and aarch64, and a container maintained by myself for armv7l. This ensures that the resulting binaries target glibc 2.17, which is required by manylinux2014.

I was unable to get cross compilation working, since devtoolset-10 does not appear to provide cross compilers. Using the system cross compiler, which is gcc 4.8.5, is unable to properly build some of the dependencies. Thus, QEMU emulation is used to build "native" aarch64 and armv7l binaries. A full cache-less build takes roughly two hours.

Other changes:

• Split out the Linux and MacOS build jobs
• Add patches to BoringSSL for it to build on old Centos 7
• Add ability to parallelize builds
• Set an env var to target MacOS SDK 10.15 (haven't inspected the binaries to see if this works yet)

Needed for lexiforest/curl_cffi#186

[bjia56]

@yifeikong can you please help rerun the CI jobs?

[perklet]

Why does the CI job need my approval to run? shouldn't this be automatic? 😂

[bjia56]

I think it's a repo setting to require approval from new contributors, not sure why it's set to requires approval by default here (maybe because the upstream had it set?)

[bjia56]

fyi, the MacOS build seems to be non-deterministically flaky on nghttpd tests, though I don't think it's related to my changes. I'd like to check out what the underlying error is though and see if it can be made more reliable

[perklet]

I see all the checks have passed, is it ready to be merged?

[bjia56]

Yes, I think it's ready. I can look into Mac test flakiness separately

[perklet]

MacOS build seems to be non-deterministically flaky on nghttpd tests

Yes, I think the original author already tried to mitigate this issue, it's much better, but not fully resolved.