[Mach-O] Support -flat_namespace flag

[carlocab]

This implements the minimum necessary to support the -flat_namespace flag.

I have not added this flag to the -help text, because I'm not certain this is faithful to ld64's behaviour under this flag. From man ld:

 -flat_namespace
         Alters how symbols are resolved at build time and runtime.  With -two_levelnamespace (the default), the linker only
         searches dylibs on the command line for symbols, and records in which dylib they were found.  With -flat_namespace, the
         linker searches all dylibs on the command line and all dylibs those original dylibs depend on.  The linker does not record
         which dylib an external symbol came from, so at runtime dyld again searches all images and uses the first definition it
         finds.  In addition, any undefines in loaded flat_namespace dylibs must be resolvable at build time.

However, I believe what is done here is enough to approximate what build systems expect to happen when they pass -flat_namespace to the linker.

Closes #785.

Signed-off-by: Carlo Cabrera [email protected]

[carlocab]

Actually, I'd like to test this some more to make sure.

[rui314]

Yeah, I think the flag needs more than this. We record dylib names along with dynamic symbols, but that doesn't make sense for the flag namespace.