py: Allow inline-assembler emitter to be generic.

This patch refactors some code so that it is easier to integrate new
inline assemblers for different architectures other than ARM Thumb.
diff --git a/py/nativeglue.c b/py/nativeglue.c
index bc2f4ff..5f2164e 100644
--- a/py/nativeglue.c
+++ b/py/nativeglue.c
@@ -64,7 +64,7 @@
 
 #endif
 
-#if MICROPY_EMIT_NATIVE || MICROPY_EMIT_INLINE_THUMB
+#if MICROPY_EMIT_NATIVE || MICROPY_EMIT_INLINE_ASM
 
 // convert a native value to a Micro Python object based on type
 mp_obj_t mp_convert_native_to_obj(mp_uint_t val, mp_uint_t type) {