scripts/checkstack.pl: update to get AArch64 port from Linux

Bring checkstack.pl up to date from its upstream Linux development.
Effectively, the following linux commits:

208ad00 checkstack.pl: port to AArch64
fda9f99 scripts/checkstack.pl: automatically handle 32-bit and 64-bit mode for ARCH=x86
7eb6e34 kbuild: trivial - remove trailing empty lines
690998b scripts/checkstack.pl: Add metag support

Reported-by: York Sun <yorksun@freescale.com>
Cc: Masahiro Yamada <yamada.m@jp.panasonic.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>