mpy-cross/main: Exit with error if arch not specified with emit=native.

Currently, mpy-cross will crash if called as:

    mpy-cross -X emit=native foo.py

because it tries to use the native emitter with no target architecture set.

Fix that by checking that an architecture is set when `-X emit=native` or
`-X emit=viper` is used.

Signed-off-by: Damien George <damien@micropython.org>

mpy-cross/main.c

@@ -350,6 +350,15 @@ MP_NOINLINE int main_(int argc, char **argv) {
         }
     }
 
+    #if MICROPY_EMIT_NATIVE
+    if ((MP_STATE_VM(default_emit_opt) == MP_EMIT_OPT_NATIVE_PYTHON
+         || MP_STATE_VM(default_emit_opt) == MP_EMIT_OPT_VIPER)
+        && mp_dynamic_compiler.native_arch == MP_NATIVE_ARCH_NONE) {
+        mp_printf(&mp_stderr_print, "arch not specified\n");
+        exit(1);
+    }
+    #endif
+
     if (input_file == NULL) {
         mp_printf(&mp_stderr_print, "no input file\n");
         exit(1);