[compiler-rt] Add siginfo into SignalContext

Summary:
Information stored there is often been passed along with SignalContext.

Part of https://github.com/google/sanitizers/issues/637

Reviewers: eugenis, alekseyshl

Subscribers: kubamracek, llvm-commits, dberris

Differential Revision: https://reviews.llvm.org/D37792

llvm-svn: 313167