Disable availability macros in libc++ on Darwin.

libc++'s headers are annotated with availability macros that indicate
which version of Mac OS was the first to ship with a libc++ feature
available in its *system's* libc++.dylib. We do not use the system's
library, but rather ship our own. As such, these availability
attributes are meaningless for us but cause build breaks when we try
to use code that would not be available in the system's dylib.

Test: removed the darwin check and checked showcommands on linux
Bug: None
Change-Id: I3d328d7363eba83f1cb857c8abfd6cd0173a3058