arm64: expand CFI shadow to cover 48-bit virtual addresses

Starting from Linux 4.7, arm64's defconfig enables 48-bit VAs, see:
https://git.kernel.org/torvalds/c/211102d8

On arm64, the CFI shadow configuration currently assumes that VAs
are 39-bit long, and as expected this results in a segfault on a
(defconfig) 4.7+ kernel, when linking a CFI-enabled library.

Consequently, this change increases the max target address to
account for the new max VA size.

Change-Id: I3fb808563fa77a457c65e9663da0613117332072