libsanitizer, Darwin : Handle missing __builtin_os_log_format.

libsanitizer, Darwin : Handle missing __builtin_os_log_format.

GCC does not, currently, define __builtin_os_log_format, which
is needed by os/log.h.  Do not include that header unless the
builtin is defined (since the header errors out on the same
condition).  Provide a work-around solution to the missing API
provided via the header.

libsanitizer/ChangeLog:

	* sanitizer_common/sanitizer_mac.cpp : Check for the
	availability of __builtin_os_log_format before trying to
	include a header depending on it.
	(OS_LOG_DEFAULT): New.
	(os_log_error): Define to a fall-back using an older API.