[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.

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@189753 91177308-0d34-0410-b5e6-96231b3b80d8