[sanitizer] First step toward supporting 42-bit AS on aarch64

aarch64-linux kernel has configurable 39, 42 or 47 bit virtual address
space.  Most distros AFAIK use 42-bit VA right now, but there are also
39-bit VA users too.  The ppc64 handling can be used for this just fine
and support all the 3 sizes.

There are other issues, like allocator32 not really being able to support
the larger addres spaces, and hardcoded 39-bit address space size in other
macros.

Patch by Jakub Jelinek.

llvm-svn: 226639