Make mostlyclean an alias for clean

Make mostlyclean an alias for clean

I found out by accident that "mostlyclean" in a sim subdir removes all
the configure artifacts.  The usual rule is:

* If the maintainer built it, maintainer-clean should remove it;
* If configure built it, distclean should remove it;
* If make built it, "clean" should remove it;
* If there is a handy subset of "clean" that is "easy" to rebuild,
  "mostlyclean" should remove it; otherwise mostlyclean should be an
  alias for clean

This patch makes mostlyclean an alias for clean.

sim/common/ChangeLog
2021-04-22  Tom Tromey  <tom@tromey.com>

	* Make-common.in (mostlyclean): Now an alias for clean, not
	distclean.