[asan] Enable 48-bit VMA support on aarch64

This patch adds 48-bits VMA support for asan on aarch64. The current
47-bit mask is not suffice since on aarch64 kernel with 48-bit vma
(default on ubuntu 16.04) the process may use full VMA range as:

[...]
ffffa39a7000-ffffa39a8000 r--p 00000000 00:00 0 [vvar]
ffffa39a8000-ffffa39a9000 r-xp 00000000 00:00 0 [vdso]
ffffa39a9000-ffffa39aa000 r--p 0001c000 08:02 13631554 /lib/aarch64-linux-gnu/ld-2.23.so
ffffa39aa000-ffffa39ac000 rw-p 0001d000 08:02 13631554 /lib/aarch64-linux-gnu/ld-2.23.so
ffffc2227000-ffffc2248000 rw-p 00000000 00:00 0 [stack]

llvm-svn: 275792