Ada: Fix incorrect handling of BOM by -r switch of gnatchop

Ada: Fix incorrect handling of BOM by -r switch of gnatchop

As reported and analyzed in the PR, gnatchop does not correctly propagate
a BOM present in the source file to the first compilation unit it outputs,
in the case where the -r switch is specified, because it copies the BOM
for the first compilation unit as part of the chopping process instead of
copying it specifically at the start of the unit.

gcc/ada/
	PR ada/81106
	* gnatchop.adb (Gnatchop): If present in the source file, output
	the BOM at the start of every compilation unit.