configure: detect Loongson MIPS extensions

Originally committed as revision 18978 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -897,6 +897,7 @@ HAVE_LIST="
     libdc1394_1
     libdc1394_2
     llrint
+    loongson
     lrint
     lrintf
     lzo1x_999_compress
@@ -1916,6 +1917,8 @@ fi
 
 # check for assembler specific support
 
+enabled mips && check_asm loongson '"dmult.g $1, $2, $3"'
+
 enabled ppc && check_asm dcbzl '"dcbzl 0, 1"'
 enabled ppc && check_asm ppc4xx '"maclhw r10, r11, r12"'
 enabled ppc && check_asm xform_asm '"lwzx 0, %y0" :: "Z"(*(int*)0)'