Improve configurability for native x64/thumb emitter.

With MICROPY_EMIT_X64 and MICROPY_EMIT_THUMB disabled, the respective
emitters and assemblers will not be included in the code.  This can
significantly reduce binary size for unix version.
diff --git a/py/emitcpy.c b/py/emitcpy.c
index 652617c..7b2d50f 100644
--- a/py/emitcpy.c
+++ b/py/emitcpy.c
@@ -13,6 +13,7 @@
 #include "runtime0.h"
 #include "emit.h"
 
+// wrapper around everything in this file
 #if MICROPY_EMIT_CPYTHON
 
 struct _emit_t {