[sanitizer] MmapAlignedOrDie changes to reduce fragmentation

Summary:
The reasoning behind this change is explained in D33454, which unfortunately
broke the Windows version (due to the platform not supporting partial unmapping
of a memory region).

This new approach changes `MmapAlignedOrDie` to allow for the specification of
a `padding_chunk`. If non-null, and the initial allocation is aligned, this
padding chunk will hold the address of the extra memory (of `alignment` bytes).
This allows `AllocateRegion` to get 2 regions if the memory is aligned
properly, and thus help reduce fragmentation (and saves on unmapping
operations). As with the initial D33454, we use a stash in the 32-bit Primary
to hold those extra regions and return them on the fast-path.

The Windows version of `MmapAlignedOrDie` will always return a 0
`padding_chunk` if one was requested.

Reviewers: alekseyshl, dvyukov, kcc

Reviewed By: alekseyshl

Subscribers: llvm-commits, kubamracek

Differential Revision: https://reviews.llvm.org/D34152

llvm-svn: 305391