Fix MIPS tests

Tests failed to build due to use of legacy const generics syntax, out-of-bounds immediates and incorrect name of const generics in `assert_instr`

Author: eduardosm

ci/docker/mips64-unknown-linux-gnuabi64/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:25.10
+FROM ubuntu:25.04 # gcc-mips64-linux-gnuabi64 not available in 25.10
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         gcc libc6-dev qemu-user ca-certificates \

ci/docker/mips64el-unknown-linux-gnuabi64/Dockerfile

@@ -1,4 +1,4 @@
-FROM ubuntu:25.10
+FROM ubuntu:25.04 # gcc-mips64el-linux-gnuabi64 not available in 25.10
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         gcc libc6-dev qemu-user ca-certificates \

crates/core_arch/src/lib.rs

@@ -70,7 +70,11 @@
 )]
 #![cfg_attr(
     test,
-    feature(stdarch_arm_feature_detection, stdarch_powerpc_feature_detection,)
+    feature(
+        stdarch_arm_feature_detection,
+        stdarch_mips_feature_detection,
+        stdarch_powerpc_feature_detection,
+    )
 )]
 
 #[cfg(test)]