[Sanitizer] Add interceptor for pthread_sigmask

Summary:
pthread_sigmask is just like sigprocmask, except that its behavior in
multithreaded programs is explicitly specified. Sanitizers were lacking
a common interceptor for pthread_sigmask (although some specific
sanitizers defined custom version), which lead to false positives
(at least in msan) when using this function.

The interceptor implementation, and its test are based on the equivalent
code for sigprocmask.

Reviewers: eugenis, vitalybuka

Subscribers: kubamracek, delcypher, jfb, jdoerfert, llvm-commits, #sanitizers

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D58382

llvm-svn: 354874