[compiler-rt] Use GetNextInstructionPc in signal handlers

Summary:
All other stack trace callers assume that PC contains return address.
HWAsan already use GetNextInstructionPc in similar code.

PR43339

Reviewers: eugenis, kcc, jfb

Subscribers: dexonsmith, dberris, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D68313

llvm-svn: 373529