py/emitndebug: Add native debug emitter.

This emitter prints out pseudo-machine instructions, instead of the usual
output of the native emitter.  It can be enabled on any port via
`MICROPY_EMIT_NATIVE_DEBUG` (make sure other native emitters are disabled)
but the easiest way to use it is with mpy-cross:

    $ mpy-cross -march=debug file.py

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

py/py.mk

@@ -116,6 +116,7 @@ PY_CORE_O_BASENAME = $(addprefix py/,\
 	emitnxtensawin.o \
 	asmrv32.o \
 	emitnrv32.o \
+	emitndebug.o \
 	formatfloat.o \
 	parsenumbase.o \
 	parsenum.o \