[libsanitizer] Use internal_fork() to spawn the symbolizer process.
This should fix https://code.google.com/p/thread-sanitizer/issues/detail?id=61


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@208707 91177308-0d34-0410-b5e6-96231b3b80d8