[scudo] Change aligned alloc functions to be more compliant & perf changes

Summary:
We were not following the `man` documented behaviors for invalid arguments to
`memalign` and associated functions. Using `CHECK` for those was a bit extreme,
so we relax the behavior to return null pointers as expected when this happens.
Adapt the associated test.

I am using this change also to change a few more minor performance improvements:
- mark as `UNLIKELY` a bunch of unlikely conditions;
- the current `CHECK` in `__sanitizer::RoundUpTo` is redundant for us in *all*
  calls. So I am introducing our own version without said `CHECK`.
- change our combined allocator `GetActuallyAllocatedSize`. We already know if
  the pointer is from the Primary or Secondary, so the `PointerIsMine` check is
  redundant as well, and costly for the 32-bit Primary. So we get the size by
  directly using the available Primary functions.

Finally, change a `int` to `uptr` to avoid a warning/error when compiling on
Android.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34782

llvm-svn: 306698