[sanitizer] Try to fix LargeMmapAllocator test on Windows

This test attempts to allocate 100 512MB aligned pages of memory. This
is implemented in the usual way by allocating size + alignment bytes and
aligning the result. As a result, this test allocates 51.2GB of memory.
Windows allocates swap for all memory allocated, and our bots do not
have this much swap available.

Avoid the failure by using a more reasonable alignment, like 16MB, as we
do on 32-bit.

llvm-svn: 276779