wasm2c warm.h: inline panic to avoid compile warning

I got the following warning when compiling zig-wasm2c:

C:\git\zig\stage1\wasm.h(55) : warning C4715: 'WasmMut_toC': not all control paths return a value

This is because the C compiler doesn't understand that the panic
function from `panic.h` is noreturn.  I addressed this copying the body
of the panic function into the call-site directly which is just a printf
and abort.  Now the compiler knows the codepath aftewards is unreachable
because abort has the propery function decorators to mark it noreturn.

Author: marler8997

stage1/wasm.h

@@ -50,7 +50,9 @@ static const char *WasmMut_toC(enum WasmMut val_type) {
     switch (val_type) {
         case WasmMut_const: return "const ";
         case WasmMut_var: return "";
-        default: panic("unsupported mut");
+        default:
+            fprintf(stderr, "unsupported mut\n");
+            abort();
     }
 }