SanitizerCommon: fixes for unwinding & backtrace on SPARC

Summary:
This patch contains various fixes for the unwinding and backtrace machinery on the SPARC, which doesn't work correctly in various cases.  It was tested with GCC on SPARC/Solaris and SPARC/Linux.

Patch by Eric Botcazou.

Reviewers: #sanitizers, vitalybuka

Reviewed By: #sanitizers, vitalybuka

Subscribers: jrtc27, delcypher, vitalybuka, ro, jyknight, kubamracek, fedor.sergeev, jdoerfert, llvm-commits, #sanitizers

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D58431

llvm-svn: 355965