scripts/checkstack.pl: Fix arm64 wrong or unknown architecture

The following error occurs for the `make ARCH=arm64 checkstack` case:

aarch64-linux-gnu-objdump -d vmlinux $(find . -name '*.ko') | \
perl ./scripts/checkstack.pl arm64
wrong or unknown architecture "arm64"

As suggested by Masahiro Yamada, fix the above error using regular
expressions in the same way it was fixed for the `ARCH=x86` case via
commit fda9f990 ("scripts/checkstack.pl: automatically handle
32-bit and 64-bit mode for ARCH=x86").

Suggested-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: George G. Davis <george_davis@mentor.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>