x86: avoid flush IPI when possible

x86: avoid flush IPI when possible

Since CLFLUSH, other than WBINVD, is a cache coherency domain wide
flush, there's no need to IPI other CPUs if this is the only flushing
being requested. (As a secondary change, move a local variable into the
scope where it's actually needed.)

As a secondary change also eliminate another leftover from 32-bit days:
invalidate_interrupt() can clear FLUSH_TLB_GLOBAL alongside FLUSH_TLB,
since write_ptbase() (as a descendant of __sync_local_execstate()) now
unconditionally fiddles with CR4.PGE.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>