xen/x86: Fix cflush()'s parameter tracking

xen/x86: Fix cflush()'s parameter tracking

Forcing a register operand hides (from the compiler) the fact that clflush
behaves as a read from the memory operand (wrt memory order, faults, etc.).
It also reduces the compilers flexibility with register scheduling.

Re-implement clfush() (and wbinvd() for consistency) as a static inline rather
than a macro, and have it take a const void pointer.

In practice, the only generated code which gets modified by this is in
mwait_idle_with_hints(), where a disp8 encoding now gets used.

While here, I noticed that &mwait_wakeup(cpu) was being calculated twice.
This is caused by the memory clobber in mb(), so take the opportunity to help
the optimiser by calculating it once, ahead of time.  bloat-o-meter reports a
delta of -26 as a result of this change.

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