[Ada] Fix indentation of "Start of processing for ..." labels

[Ada] Fix indentation of "Start of processing for ..." labels

The "Start of processing for ..." labels should be aligned with the
BEGIN that follows. Violations found with:

$ grep -Pzo "( *)   \-\-  Start of processing for \w+\n\n\1begin" *.adb

and corrected manually, because some of them had to be entirely removed.

Also, remove trailing semicolons found while guessing the grep pattern.

gcc/ada/

	* exp_aggr.adb, exp_ch3.adb, exp_util.adb, freeze.adb,
	gnatfind.adb, gnatlink.adb, gnatxref.adb, gprep.adb,
	libgnat/a-cfhama.adb, libgnat/g-alleve.adb, sem_ch12.adb,
	sem_ch13.adb, xr_tabls.adb: Fix style labels.