x86/traps: Clean up printing in {do_reserved,fatal}_trap()

x86/traps: Clean up printing in {do_reserved,fatal}_trap()

For one, they render the vector in a different base.

Introduce X86_EXC_* constants and vec_name() to refer to exceptions by their
mnemonic, which starts bringing the code/diagnostics in line with the Intel
and AMD manuals.

Provide constants for every archtiecturally defined exception, even those not
implemented by Xen yet, as do_reserved_trap() is a catch-all handler.

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