xen/arm32: entry: Split __DEFINE_ENTRY_TRAP in two

xen/arm32: entry: Split __DEFINE_ENTRY_TRAP in two

The preprocessing macro __DEFINE_ENTRY_TRAP is used to generate trap
entry function. While the macro is fairly small today, follow-up patches
will increase the size signicantly.

In general, assembly macros are more readable as they allow you to name
parameters and avoid '\'. So the actual implementation of the trap is
now switched to an assembly macro.

This is part of XSA-303.

Reported-by: Julien Grall <Julien.Grall@arm.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>