xen/arm: Rework psr_mode_is_32bit()

xen/arm: Rework psr_mode_is_32bit()

psr_mode_is_32bit() prototype does not match the rest of the helpers for
the process state. Looking at the callers, most of them will access
struct cpu_user_regs just for calling psr_mode_is_32bit().

The macro is now reworked to take a struct cpu_user_regs in parameter.
At the same time take the opportunity to switch to a static inline
helper.

Lastly, when compiled for 32-bit, Xen will only support 32-bit guest. So
it is pointless to check whether the register state correspond to 64-bit
or not.

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