[scudo] Lay the foundation for 32-bit support

Summary:
In order to support 32-bit platforms, we have to make some adjustments in
multiple locations, one of them being the Scudo chunk header. For it to fit on
64 bits (as a reminder, on x64 it's 128 bits), I had to crunch the space taken
by some of the fields. In order to keep the offset field small, the secondary
allocator was changed to accomodate aligned allocations for larger alignments,
hence making the offset constant for chunks serviced by it.

The resulting header candidate has been added, and further modifications to
allow 32-bit support will follow.

Another notable change is the addition of MaybeStartBackgroudThread() to allow
release of the memory to the OS.

Reviewers: kcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D25688

llvm-svn: 285209