[libsanitizer] Make sanitizer_unwind_posix.cc a libcdep source file.
Guard the source with #if SANITIZER_POSIX to fix compilation on Windows.
use it only in 


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@216882 91177308-0d34-0410-b5e6-96231b3b80d8