[hwasan] Fix allocator alignment.

Summary:
Fix hwasan allocator not respecting the requested alignment when it is
higher than a page, but still within primary (i.e. [2048, 65536]).

Reviewers: pcc, hctim, cryptoad

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D79656