[PATCH] x86_64: (resend) x86_64 stack overflow debugging

Take two, now without spurious whitespace :(  Applies to git & 2.6.17-rc6

CONFIG_DEBUG_STACKOVERFLOW existed for x86_64 in 2.4, but seems to have gone AWOL in 2.6.

I've pretty much just copied this over from the 2.4 code, with
appropriate tweaks for the 2.6 kernel, plus a bugfix.  I'd personally
rather see it printed out the way other arches do it, i.e.
bytes-remaining-until-overflow, rather than having to do the subtraction
yourself.  Also, only 128 bytes remaining seems awfully late to issue a
warning.  But I'll start here :)

Signed-off-by: Eric Sandeen <sandeen@sgi.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>