x86: fix paravirt clobber in entry_64.S

Impact: Fix latent bug

The clobber is trying to say that anything except RDI is available for
clobbering, but actually clobbers everything.  This hasn't mattered
because the clobbers were basically ignored, but subsequent patches
will rely on them.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>