configure: [loongson] adjust MMI check in configure

After standardizing the use of 'pxor' in commit 'ebedd26', FFmpeg
build failed with upstream compiler, for 'pxor' is not supported
in time. This patch helps to workaround the build failure by
checking whether 'pxor' is supported during configuration, if not,
MMI will be disabled.

Reviewed-by: yinshiyou-hf@loongson.cn
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

configure

@@ -5947,7 +5947,7 @@ elif enabled mips; then
     enabled loongson3 && check_inline_asm loongson3 '"gsldxc1 $f0, 0($2, $3)"' '-mloongson-ext' && append MMIFLAGS '-mloongson-ext'
 
     # MMI can be detected at runtime too
-    enabled mmi && check_inline_asm mmi '"punpcklhw $f0, $f0, $f0"' '-mloongson-mmi' && append MMIFLAGS '-mloongson-mmi'
+    enabled mmi && check_inline_asm mmi '"pxor $f0, $f0, $f0"' '-mloongson-mmi' && append MMIFLAGS '-mloongson-mmi'
 
     if enabled bigendian && enabled msa; then
         disable msa