[sanitizer] Use system unwinder in signal handlers on Android.

Because of the way Bionic sets up signal stack frames, libc unwinder is unable
to step through it, resulting in broken SEGV stack traces.

Luckily, libcorkscrew.so on Android implements an unwinder that can start with
a signal context, thus sidestepping the issue.


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@201151 91177308-0d34-0410-b5e6-96231b3b80d8