[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