Bug 1648697 - Disable --enable-avx. r=bbouvier

I disable this with an error message and a comment, but do not remove it,
because it is currently buggy and unsupported but may well make a comeback
when we have time to clean it up and we see whether wasm SIMD on the web
starts demanding it.

It is of course possible that we will never do this in Ion but will wait
for Cranelift, but we don't have to make that decision today.

Differential Revision: https://phabricator.services.mozilla.com/D81293

js/src/shell/fuzz-flags.txt

@@ -72,8 +72,6 @@
 --wasm-disable-huge-memory
 
 # CPU instruction set-related
---enable-avx
---no-avx
 --no-sse3
 --no-ssse3
 --no-sse41

js/src/shell/js.cpp

@@ -11221,6 +11221,10 @@ int main(int argc, char** argv, char** envp) {
     if (!sCompilerProcessFlags.append("--enable-avx")) {
       return EXIT_FAILURE;
     }
+    // Disable AVX completely for now.  We're not supporting AVX and things
+    // break easily if asking for AVX.
+    fprintf(stderr, "Error: AVX encodings are currently disabled\n");
+    return EXIT_FAILURE;
   }
 #endif