[msan] Intercept __getrlimit.

Summary:
This interceptor is useful on its own, but the main purpose of this
change is to intercept libpthread initialization on linux/glibc in
order to run __msan_init before any .preinit_array constructors.

We used to trigger on pthread_initialize_minimal -> getrlimit(), but
that call has changed to __getrlimit at some point.

Reviewers: vitalybuka, pcc

Subscribers: jfb, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D68168

llvm-svn: 373239