scripts/checkstack.pl: automatically handle 32-bit and 64-bit mode for ARCH=x86

This patch adds support for ARCH=x86 into checkstack.

Commit ffee0de4

 ("x86: Default to ARCH=x86 to avoid overriding
CONFIG_64BIT") had merged ARCH=i386 and ARCH=x86_64 into one ARCH=x86.
checkstack.pl searches patterns of machine instructions which are
usually used for allocating stack frames.  checkstalk.pl needs either
i386 or x86_64, x86 isn't enough:

  $ make checkstack
  objdump -d vmlinux $(find . -name '*.ko') | \
  perl linux/scripts/checkstack.pl x86
  wrong or unknown architecture "x86"

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>