x86/hypercall: Make the HVM hcall_64bit boolean common

x86/hypercall: Make the HVM hcall_64bit boolean common

HVM guests currently make use of arch.hvm_vcpu.hcall_64bit to track the ABI of
the hypercall in use.

The rest of Xen deals in terms of the comat ABI or not, so rename the boolean
and make it common, guared by CONFIG_COMPAT to avoid bloat if a compat ABI is
not wanted/needed.

Set hcall_compat uniformly for PV guests as well as HVM guests.  This removes
the remaining piece of guest-type-specific knowledge from
hypercall_create_continuation(), allowing it to operate only in terms of the
hypercall ABI in use.

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