[ASan] Small refactoring - split the slow unwinding logic into sanitizer_unwind_posix.cc
This allows the slow unwinder to be used on OSX (it's disabled for now, see
https://code.google.com/p/address-sanitizer/issues/detail?id=137 for details)


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@216877 91177308-0d34-0410-b5e6-96231b3b80d8