asan: fix crash on odd stack size

The test currently crashes as:

AddressSanitizer: CHECK failed: asan_poisoning.cpp:38 "((AddrIsAlignedByGranularity(addr))) != (0)"

Main stack address/size don't have to be aligned on asan shadow granularity.
Align stack bottom.

Reviewed By: melver, vitalybuka

Differential Revision: https://reviews.llvm.org/D145799