hypercall: Work around Clang code generation bug

hypercall: Work around Clang code generation bug

_hypercall64_4() is mis-compiled by some versions of Clang (5 is ok, 9 and
later not), in a way which breaks the XSA-286 PoC.

Clang loses the association between argument 4 being in %r10 between the input
"a" (_a4) and the output "=&r" (__ign4), so while the input is handled
correctly, the clobber on the output is missed.

In the XSA-286, PoC, this causes clang to hoist the setting of DOMID_SELF out
of the loop, and the test subsequently to malfunction on debug versions of Xen
which intentionally clobber the hypercall parameters.

Rewrite the hypercall macros to a single "+r" parameter form, rather than dual
input/output form, which causes all tested versions of Clang to set up %r10 on
each loop iteration.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>