Add DemangleFunctionName for backtracing on NetBSD

Summary:
NetBSD uses indirection for old threading functions for historical reasons
The mangled names are internal implementation detail and should not be
exposed even in backtraces.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka, dvyukov

Reviewed By: dvyukov

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D40251

llvm-svn: 318671