Mmap interceptor providing mprotect support

Summary:
- Intercepting mprotect calls.
- Fixing forgotten flag check.

Patch by David CARLIER

Reviewers: vitalybuka, vsk

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

Differential Revision: https://reviews.llvm.org/D44777

llvm-svn: 328415