un-hardcode sanitizer Mac OS X min-version, requires llvm commit fa77121
more precise use of #pragma pack(4) for struct __sanitizer_ifconf
works on darwin8,9
eliminate redundant definition of __DARWIN_VERSION__ in asan_mac.cc

Conflicts:
	lib/sanitizer_common/sanitizer_platform_limits_posix.h
don't try to provide cache invalidation, use libSystem's instead
test template for libclang_rt
helper script for assembling and linking .S files into libclang_rt
needs OS X version info from header