[scudo][standalone] Add a free list to the Secondary

Summary:
The secondary allocator is slow, because we map and unmap each block
on allocation and deallocation.

While I really like the security benefits of such a behavior, this
yields very disappointing performance numbers on Android for larger
allocation benchmarks.

So this change adds a free list to the secondary, that will hold
recently deallocated chunks, and (currently) release the extraneous
memory. This allows to save on some memory mapping operations on
allocation and deallocation. I do not think that this lowers the
security of the secondary, but can increase the memory footprint a
little bit (RSS & VA).

The maximum number of blocks the free list can hold is templatable,
`0U` meaning that we fallback to the old behavior. The higher that
number, the higher the extra memory footprint.

I added default configurations for all our platforms, but they are
likely to change in the near future based on needs and feedback.

Reviewers: hctim, morehouse, cferris, pcc, eugenis, vitalybuka

Subscribers: mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D69570