libacpi: don't build x86-only AML for ARM64 mk_dsdt

libacpi: don't build x86-only AML for ARM64 mk_dsdt

Commit d6ac8e22c7c5 ("acpi/x86: define ACPI IO registers for
PVH guests") broke ARM64 build of mk_dsdt.c due to introduction
of XEN_ACPI_CPU_MAP[_LEN] macros that are needed only for x86
guests.

We could fix the build by dealing specifically with those macros
but since post-MADT code is not executed on ARM64 anyway we can
compile it for x86 only.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>