ada: Fix style in lines starting with assignment operator

ada: Fix style in lines starting with assignment operator

Style cleanup; semantics is unaffected. Offending occurrences found with
grep "^ *:=" and fixed manually.

gcc/ada/

	* checks.ads, cstand.adb, exp_aggr.adb, exp_ch4.adb, exp_ch5.adb,
	exp_dbug.adb, exp_util.adb, gnatlink.adb, lib-util.adb,
	libgnat/a-except.adb, libgnat/a-exexpr.adb, libgnat/a-ngcoar.adb,
	libgnat/s-rannum.adb, libgnat/s-trasym__dwarf.adb, osint.adb,
	rtsfind.adb, sem_case.adb, sem_ch12.adb, sem_ch13.adb,
	sem_ch3.adb, sem_ch6.adb, sem_eval.adb, sem_prag.adb,
	sem_util.adb: Fix style.