configure: detect 64-bit MIPS

configure: detect 64-bit MIPS

While right now 64-bit MIPS and 32-bit MIPS share the code in QEMU,
Rust uses different rules for the target.  Set $cpu correctly to
either mips or mips64 (--cpu=mips64* is already accepted in the case
statement that canonicalizes cpu/host_arch/linux_arch), and adjust
the checks to account for the different between $cpu (which handles
mips/mips64 separately) and $host_arch (which does not).

Fixes: 1a6ef6ff624 ("configure, meson: detect Rust toolchain", 2024-10-11)
Acked-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>