xen64: fix calls into hypercall page

The 64-bit calling convention for hypercalls uses different registers
from 32-bit.  Annoyingly, gcc's asm syntax doesn't have a way to
specify one of the extra numeric reigisters in a constraint, so we
must use explicitly placed register variables.  Given that we have to
do it for some args, may as well do it for all.

Also fix syntax gcc generates for the call instruction itself.  We
need a plain direct call, but the asm expansion which works on 32-bit
generates a rip-relative addressing mode in 64-bit, which is treated
as an indirect call.  The alternative is to pass the hypercall page
offset into the asm, and have it add it to the hypercall page start
address to generate the call.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>