[tsan] Use re-exec method to enable interceptors on older versions of OS X

In AddressSanitizer, we have the MaybeReexec method to detect when we're running without DYLD_INSERT_LIBRARIES (in which case interceptors don't work) and re-execute with the environment variable set. On OS X 10.11+, this is no longer necessary, but to have ThreadSanitizer supported on older versions of OS X, let's use the same method as well. This patch moves the implementation from `asan/` into `sanitizer_common/`.

Differential Revision: http://reviews.llvm.org/D15123

llvm-svn: 254600