x86: Clean up asm keyword usage (asm volatile rather than __asm__

x86: Clean up asm keyword usage (asm volatile rather than __asm__
__volatile__ in most places) and ensure we use volatile keyword
wherever we have an asm stmt that produces outputs but has other
unspecified side effects or dependencies other than the
explicitly-stated inputs.

Also added volatile in a few places where its not strictly necessary
but where it's unlikely to produce worse code and it makes our
intentions perfectly clear.

The original problem this patch fixes was tracked down by Joseph
Cihula <joseph.cihula@intel.com>.

Signed-off-by: Keir Fraser <keir@xensource.com>