x86/smp: move and clean APIC helpers

x86/smp: move and clean APIC helpers

Move __prepare_ICR{2}, apic_wait_icr_idle and
__default_send_IPI_shortcut to the top of the file, since they will be
used by send_IPI_mask in future changes.

While there, take the opportunity to remove the leading underscores,
drop the inline attribute, drop the default prefix from the shorthand
helper, change the return type of the prepare helpers to unsigned and
do some minor style cleanups.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>