[esan|wset] Ensure SIGSEGV is not blocked

Summary:
Adds interception of sigprocmask and pthread_sigmask to esan so that the
working set tool can prevent SIGSEGV from being blocked.  A blocked SIGSEGV
results in crashes due to our lazy shadow page allocation scheme.

Adds new sanitizer helper functions internal_sigemptyset and
internal_sigismember.

Adds a test to workingset-signal-posix.cpp.

Reviewers: aizatsky

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

Differential Revision: http://reviews.llvm.org/D22063

llvm-svn: 274672