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: 1a6ef6ff62 ("configure, meson: detect Rust toolchain", 2024-10-11)
Acked-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -395,7 +395,11 @@ elif check_define _ARCH_PPC ; then
     cpu="ppc"
   fi
 elif check_define __mips__ ; then
-  cpu="mips"
+  if check_define __mips64 ; then
+    cpu="mips64"
+  else
+    cpu="mips"
+  fi
 elif check_define __s390__ ; then
   if check_define __s390x__ ; then
     cpu="s390x"
@@ -1230,7 +1234,7 @@ EOF
       fi
     fi
 
-    case "$host_arch" in
+    case "$cpu" in
     arm)
       # e.g. arm-unknown-linux-gnueabi, arm-unknown-linux-gnueabihf
       write_c_skeleton
@@ -1278,7 +1282,7 @@ EOF
     test "$rust_arch" = arm && test "$rust_os" != linux && rust_arch=armv7
     ;;
 
-  mips|mips64)
+  mips)
     # preserve ISA version (mipsisa64r6 etc.) and include endianness
     rust_arch=${raw_cpu%el}
     test "$bigendian" = no && rust_arch=${rust_arch}el