configure: Add wasm as a fake arch

And add wasm simd128 flag, so we can add simd128 optimizations.
It can be enabled by put -msimd128 to extra cflags. There is
no runtime detection on simd128 support yet. I think that needs to
be done with JavaScript.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavutil/tests/cpu.c

@@ -104,6 +104,8 @@ static const struct {
     { AV_CPU_FLAG_RVV_F64,   "zve64d"     },
     { AV_CPU_FLAG_RV_ZVBB,   "zvbb"       },
     { AV_CPU_FLAG_RV_MISALIGNED, "misaligned" },
+#elif ARCH_WASM
+    { AV_CPU_FLAG_SIMD128,   "simd128"    },
 #endif
     { 0 }
 };