Add MIPS SIMD Arch (MSA) optimized MirrorRow function

libyuv.gyp

Index: libyuv.gyp
diff --git a/libyuv.gyp b/libyuv.gyp
index abd012ac68a52a7a56237f4c45a5ab4d01f7ecbd..7c6c80466cf67294ebfded2787123c6834b5db83 100644
--- a/libyuv.gyp
+++ b/libyuv.gyp
@@ -26,12 +26,18 @@
     # Link-Time Optimizations.
     'use_lto%': 0,
     'build_neon': 0,
+    'build_msa': 0,
     'conditions': [
        ['(target_arch == "armv7" or target_arch == "armv7s" or \
        (target_arch == "arm" and arm_version >= 7) or target_arch == "arm64")\
        and (arm_neon == 1 or arm_neon_optional == 1)', {
          'build_neon': 1,
        }],
+       ['(target_arch == "mipsel" or target_arch == "mips64el")\
+       and (mips_msa == 1)',
+       {
+         'build_msa': 1,
+       }],
     ],
   },
 
@@ -79,6 +85,11 @@
             }],
           ],
         }],
+        ['build_msa != 0', {
+          'defines': [
+            'LIBYUV_MSA',
+          ],
+        }],
         ['OS != "ios" and libyuv_disable_jpeg != 1', {
           'defines': [
             'HAVE_JPEG'