Add the mill wrapper (millw)

[Flowdalic]

To make the project easier to build add the mill wrapper (millw) submodule and make the mill executable selectable via the MILL Makefile variable. Furthermore, set ./millw as the default for the MILL Makefile variable, so that the wrapper is used as default.

[XiangShanRobot]

[Generated by IPC robot]
commit: 09720f7

commit	astar	copy_and_run	coremark	gcc	gromacs	lbm	linux	mcf	microbench	milc	namd	povray	wrf	xalancbmk
09720f7	1.885	0.461	2.100	1.204	2.904	2.175	2.215	0.918	1.421	1.127	3.611	2.647	2.300	3.176

master branch:

commit	astar	copy_and_run	coremark	gcc	gromacs	lbm	linux	mcf	microbench	milc	namd	povray	wrf	xalancbmk
692e2fa
1d97d64	1.875	0.461	2.100	1.204	2.904	2.175	2.215	0.912	1.421	1.125	3.611	2.648	2.300	3.176
3953b70	1.885	0.461	2.100	1.194	2.904	2.181	2.218	0.919	1.421	1.124	3.611	2.660	2.300	3.176
4a8a734	1.885	0.461	2.100	1.204	2.904	2.175	2.215	0.918	1.421	1.125	3.611	2.647	2.300	3.176
5b0f002	1.875	0.461	2.100	1.203	2.904	2.180	1.395	0.912	1.421	1.127	3.611	2.659	2.300	3.176
6ade72d	1.885	0.461	2.100	1.194	2.904	2.175	1.395	0.918	1.398	1.125	3.611	2.648	2.300	3.176
672c464	1.885	0.461	2.100	1.199	2.904	2.175	1.396	0.912	1.398	1.125	3.611	2.647	2.300	3.176
321934c	1.885	0.461	2.100	1.194	2.904	2.175	1.396	0.919	1.421	1.125	3.611	2.659	2.300	3.176
4176c33	1.885	0.461	2.100	1.199	2.904	2.180	1.396	0.918	1.421	1.125	3.611	2.647	2.300	3.176
a5b77de	1.875	0.461	2.100	1.195	2.904	2.180	1.396	0.919	1.421	1.127	3.611	2.658	2.300	3.176

[Tang-Haojin]

We may first merge H extension to make things simpler.

[Tang-Haojin]

We prefer downloading mill(w) in Makefile to somewhere like ~/.cache/millw. Adding a new submodule may introduce more using and maintainance complexity.

[Flowdalic]

The mill wrapper is standalone. You don't have to use a submodule but could add the wrapper files directly. Alternatively, and probably more git-idiomatic, the millw repo could be added as git subtree (instead of a submodule).