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

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

This patch adds support for ARCH=x86 into checkstack.

Commit ffee0de411fd ("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 <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Richard Weinberger <[email protected]>
Cc: Geert Uytterhoeven <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>