xen/arm64: entry: Avoid open-coding interrupt flags

xen/arm64: entry: Avoid open-coding interrupt flags

At the moment, the interrupts to mask/unmask are hardcoded in the code
making more difficult to find out what's going on.

A new series of short-hand specific to the file entry.S is now added.

The name of the short-hands should tell which interrupts will be
changed by the msr daif{set, clr} instructions.

Take the opportunity to replace the hardcoded values with the new
short-hands.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Juergen Gross <jgross@suse.com>