[Sanitizers] Share some stack walking code between Windows and Linux

Reviewed at http://llvm-reviews.chandlerc.com/D2126

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@194326 91177308-0d34-0410-b5e6-96231b3b80d8
Remove a FIXME now that I can't reproduce the problem

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@194331 91177308-0d34-0410-b5e6-96231b3b80d8