Chapel Makefiles should declare `FORCE` as a phony target

[bradcray]

As Dan @bonachea points out in #26448 (comment), Chapel Makefiles rely heavily on FORCE as a fake pseudo-target to require a Makefile step to execute. However, this approach has the fragility that if a file named FORCE happened to exist, it would not work as intended. The right way to improve this situation would be to add the line

.PHONY: FORCE

to our Makefiles, and in practice this probably should be pretty easy given that many of the Makefiles include a common footer that could contain this declaration.

This issue hasn't bitten us (that we're aware of) but is worth cleaning up, so I'm opening this issue to log it.