x86: clean up physid_mask_t handling

x86: clean up physid_mask_t handling

This eliminates passing and returning by value of this type (making it
unnecessary for the compiler to create temporary variables for doing so
on the stack), thus dramatically reducing the stack frame sizes of a
couple of functions (was in one case over 12k for a 4095-CPU build).

In one case a local variable gets converted to a static one, possible
because the function gets called at most once (during early boot).

Some accessors get eliminated altogether as being unused or as being
equally well open coded at the place of use.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>