Enable force-soft-floats for extensive tests

Any architecture-specific float operations are likely to consist of only
a few instructions, but the softfloat implementations are much more
complex. Ensure this is what gets tested.

Author: tgross35

.github/workflows/main.yaml

@@ -270,7 +270,7 @@ jobs:
           fi
 
           LIBM_EXTENSIVE_TESTS="$CHANGED" cargo t \
-            --features build-mpfr,unstable \
+            --features build-mpfr,unstable,force-soft-floats \
             --profile release-checked \
             -- extensive
       - name: Print test logs if available