[SectionMemoryManager] Abstract out mmap, munmap, mprotect even more ; NFC

Summary:
This will let ORC JIT clients plug in custom logic for the mmap, munmap and
mprotect paths.

Reviewers: loladiro, dblaikie

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D39300

llvm-svn: 317770