[compiler-rt] Fix 16-byte alignment for vec allocators on AIX (PR #184967)

[Thurston Dang via llvm-commits]

thurstond wrote:

> @thurstond any suggestions on how to test the `DlsymAlloc` path which seems to fix the alignment. I can modify current test to verify` __linux_vec*` changes

compiler-rt/test/sanitizer_common/TestCases/dlsym_alloc.c shows one way to invoke the DlsymAlloc path

https://github.com/llvm/llvm-project/pull/184967