x86: Use enum instead of literals for trap values [PARTIAL]

[Based on commit c94082656dac74257f63e91f78d5d458ac781fa5 upstream, only
taking the traps.h portion.]

The traps are referred to by their numbers and it can be difficult to
understand them while reading the code without context. This patch adds
enumeration of the trap numbers and replaces the numbers with the correct
enum for x86.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/20120310000710.GA32667@www.outflux.net
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Robin Holt <holt@sgi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>