arm: Use move-if-change for updating regenerated files [PR93548]

arm: Use move-if-change for updating regenerated files [PR93548]

The t-arm make fragment currently uses 'mv' to update some files that
are automatically regenerated, but this causes problems on read-only
filesystems if the date stamps are incorrect and the files have not
really changed.  So use move-if-change instead.

	PR target/93548
	* config/arm/t-arm: ($(srcdir)/config/arm/arm-tune.md,
	$(srcdir)/config/arm/arm-tables.opt): Use move-if-change.