x86/traps: honor EXT bit in error codes

x86/traps: honor EXT bit in error codes

The specification does not explicitly limit the use of this bit to
exceptions that can have selector style error codes, so to be on the
safe side we should deal with it being set even on error codes formally
documented to be always zero (if they're indeed always zero, the change
is simply dead code in those cases).

Introduce and use (where suitable) X86_XEC_* constants to make the code
easier to read.

To match the placement of the "hardware_trap" label, the "hardware_gp"
one gets moved slightly too.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>