[scudo] Implement and enable MemMapLinux

Most of the implementations are copied from linux.cpp and we will be
keeping those memory functions in linux.cpp for a while until we are
able to switch to use MemMap completely.

The remaining part is SizeClassAllocator32 which hasn't been switched to
use MemMap interface

Reviewed By: cferris

Differential Revision: https://reviews.llvm.org/D146453