meson.build: fix building of Xen support for aarch64

meson.build: fix building of Xen support for aarch64

Xen is supported on ARM although weirdly using the i386-softmmu model.
Checking based on the host CPU meant we never enabled Xen support. It
would be nice to enable CONFIG_XEN for aarch64-softmmu to make it not
seem weird but that will require further build surgery.

Fixes: 8a19980e3f ("configure: move accelerator logic to meson")
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu@linaro.org>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Paul Durrant <paul@xen.org>
Message-Id: <20201110192316.26397-4-alex.bennee@linaro.org>