[scudo] Make some tests less Linux-y

Summary:
Start making the Scudo tests less Linux-y:
- `malloc_usable_size` doesn't exist everywhere, so replace them with
  `__sanitizer_get_allocated_size` which we provide;
- move all the `memalign` related tests into `memalign.c` since it's also not
  available everywhere.

I also noticed that the `memalign.c` was missing a line in one of the loops.

Reviewers: alekseyshl

Reviewed By: alekseyshl

Subscribers: delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D43393

llvm-svn: 326100