Add Darwin (macOS, iOS) support

Make libglvnd able to compile on/to Darwin systems

Changes:

• Mach-O compatible assembly
• configure.ac now checks for linker flag --version-script and -Bsymbolic
• meson.build won't add --version-script and -Bsymbolic when targeting Darwin
• Using alternative semaphore API calls (mach/semaphore.h) for tests instead of unimplemented sem_init and others

Current status:

• Compiles
• Testsuite compiles
• All test passes (Currently only few of them passed)

TODO:

• Fix issues within test suites (Need help!)
• Need someone to confirm those Mach-O assembly was correct