[esan] Intercept and chain signal handlers

Summary:
In preparation for fault-based shadow memory iteration, we add support for
our own signal handler by adding app signal handler interception as well as
chaining for SIGSEGV.  This is done in a simple manner: we do not honor the
app's alternate stack nor any sigaction flags for SIGSEGV.

Adds a new test of transparency in app signal handling.

Reviewers: aizatsky

Subscribers: filcab, kubabrecka, vitalybuka, zhaoqin, kcc, eugenis, llvm-commits

Differential Revision: http://reviews.llvm.org/D20577

llvm-svn: 271272