avoid vectors for pnacl which cause linker failure.

R=sergeyu@chromium.org
BUG=chromium:538243

Review URL: https://codereview.chromium.org/1396363004 .

include/libyuv/row.h

@@ -384,7 +384,7 @@ typedef __declspec(align(32)) int8 lvec8[32];
 typedef __declspec(align(32)) uint16 ulvec16[16];
 typedef __declspec(align(32)) uint32 ulvec32[8];
 typedef __declspec(align(32)) uint8 ulvec8[32];
-#elif defined(__GNUC__)
+#elif defined(__GNUC__) && !defined(__pnacl__)
 // Caveat GCC 4.2 to 4.7 have a known issue using vectors with const.
 #define SIMD_ALIGNED(var) var __attribute__((aligned(16)))
 #define SIMD_ALIGNED32(var) var __attribute__((aligned(64)))