libsanitizer, Darwin, Bootstrap : Fix bootstrap on Darwin <= 15.

libsanitizer, Darwin, Bootstrap : Fix bootstrap on Darwin <= 15.

The latest upstream merge for libsanitizer introduces code that makes
use of some macro values that are not available in SDKs for versions
of Darwin <= 15 (macOS 10.11).

Add definitions for these where they are not present.

libsanitizer/ChangeLog:

	* sanitizer_common/sanitizer_mac.h: Ensure that TARGET_OS_
	macros are defined where the macOS SDK does not contain
	them.
	(TARGET_OS_OSX, TARGET_OS_IOS, TARGET_OS_TV, TARGET_OS_WATCH):
	Define where needed.