meson: check cpu_family() instead of cpu()

meson documents the values returned by cpu_family() as stable,
but not those returned by cpu():
https://mesonbuild.com/Reference-tables.html#cpu-families
https://mesonbuild.com/Reference-manual_builtin_host_machine.html

We already use cpu_family() in most instances, but had missed a few.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <https://gitlab.freedesktop.org/xorg/xserver/-/merge_requests/2298>
(cherry picked from commit 71c7824e80)
(cherry picked from commit acb7237a3a0e7a0425b376b58f89838668c86d2b)
(cherry picked from commit 52548625e1aacfdef5a6baccfb39f81ab818d578)

meson.build

@@ -218,7 +218,6 @@ else
     build_seatd_libseat = get_option('seatd_libseat') == 'true'
 endif
 
-
 if get_option('systemd_logind') == 'auto'
     build_systemd_logind = build_udev_kms and dbus_dep.found() and not build_seatd_libseat
 else