[sanitizer_common] Add internal_clone().

Add a wrapper for the clone syscall for use in StopTheWorld. We
implement it only for x86_64, so stop building StopTheWorld for other platforms
(no one uses it outside x86_64 anyway).

See https://code.google.com/p/address-sanitizer/issues/detail?id=214 for why we
can't use the glibc clone() wrapper.