bug#48117: 28.0.50; Update of loaddefs.el during normal build is unreliable

[Glenn Morris]

One idea, don't know if workable:
always regenerate loaddefs, but using a temporary file name for the
main loaddefs.
use build-aux/move-if-change to only replace the real loaddefs file if
there have been changes. Although again, it's possible these would not
be "real" (ie significant) changes.

This would mean every invocation of make would generate a loaddefs file,
often for no need, which seems ugly.
It's a trade off between doing things properly (which is a bootstrap)
and efficiently, as has always been the case with Emacs's build.