[NFC][Sanitizer] Weak linkage is not available on Windows

The concept of weak linkage is not available on Windows. The available
workarounds in LLVM/sanitizer runtimes have their own problems. Define a
separte symbol ubsan_GetStackTrace to work around the issue now. At lest
this way it is painfully obvious that we still have to do more cleanup.

Follow-up to revision: https://reviews.llvm.org/D58651

llvm-svn: 355113