msan: Enable 48-bit VMA support on aarch64

This patch adds 48-bits VMA support for msan on aarch64. As current
mappings for aarch64, 48-bit VMA also supports PIE executable. The
48-bits segments only cover the usual PIE/default segments plus some
more segments (262144GB total, 0.39% total VMA). Memory avaliability
can be increase by adding multiple application segments like 39 and
42 mapping (some mappings were added on this patch as well).

Tested on 39 and 48-bit VMA kernels on aarch64.

llvm-svn: 279752