[compiler-rt] Implement getrandom interception

Summary:
Straightforward implementation of `getrandom` syscall and libc
hooks.

Test Plan: Local MSAN failures caused by uninstrumented `getrandom`
calls stop failing.

Patch by Andrew Krieger.

Reviewers: eugenis, vitalybuka

Reviewed By: vitalybuka

Subscribers: srhines, kubamracek, dberris, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D65551

llvm-svn: 367999