meson: Unify mips and mips64 in host_arch

Fixes the build on a mips64 host.  Prior to the break, we identified
the arch via the __mips__ define; afterward we use meson's
host_machine.cpu_family().  Restore the previous combination.

Fixes: 823eb013 ("configure, meson: move ARCH to meson.build")
Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>