[GWP-ASan] Update alignment on Android.

Summary:
Android has different alignment requirements. You can read more about
them here
(https://cs.android.com/android/platform/superproject/+/master:bionic/tests/malloc_test.cpp;l=808),
but the general gist is that for malloc(x <= 8), we do malloc(8), and
for everything else, we do 16-byte alignment.

Reviewers: eugenis, morehouse, cferris

Reviewed By: eugenis, morehouse

Subscribers: #sanitizers, llvm-commits, pcc

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D74364