[libsanitizer] Add register dumping support to SuspendedThreadsList.

An interface for obtaining register contexts from suspended threads. Tailored
for LSan use.

Patch by Sergey Matveev (earthdok@google.com)


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@178461 91177308-0d34-0410-b5e6-96231b3b80d8