xen/arm: traps: Avoid using BUG_ON() in _show_registers()

xen/arm: traps: Avoid using BUG_ON() in _show_registers()

At the moment, _show_registers() is using a BUG_ON() to assert only
userspace will run 32-bit code in a 64-bit domain.

Such extra precaution is not necessary and could be avoided by only
checking the CPU mode to decide whether show_registers_64() or
show_reigsters_32() should be called.

This has also the nice advantage to avoid nested if in the code.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>