/usr/src/gcc-5/debian/locale-gen is in gcc-5-source 5.3.1-14ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 | #!/bin/sh
# generate locales that the libstdc++ testsuite depends on
LOCPATH=`pwd`/locales
export LOCPATH
[ -d $LOCPATH ] || mkdir -p $LOCPATH
[ -n "$USE_CPUS" ] || USE_CPUS=1
umask 022
echo "Generating locales..."
xargs -L 1 -P $USE_CPUS -I{} \
    sh -c '
	set {}; locale=$1; charset=$2
	case $locale in \#*) exit;; esac
	[ -n "$locale" -a -n "$charset" ] || exit
	echo "  `echo $locale | sed \"s/\([^.\@]*\).*/\1/\"`.$charset`echo $locale | sed \"s/\([^\@]*\)\(\@.*\)*/\2/\"`..."
        if [ -f $LOCPATH/$locale ]; then
	    input=$locale
	else
            input=`echo $locale | sed "s/\([^.]*\)[^@]*\(.*\)/\1\2/"`
	fi
	localedef -i $input -c -f $charset $LOCPATH/$locale #-A /etc/locale.alias
    ' <<EOF
de_DE			ISO-8859-1
de_DE@euro		ISO-8859-15
en_HK			ISO-8859-1
en_PH			ISO-8859-1
en_US			ISO-8859-1
en_US.ISO-8859-1	ISO-8859-1
en_US.ISO-8859-15	ISO-8859-15
en_US.UTF-8		UTF-8
es_ES			ISO-8859-1
es_MX			ISO-8859-1
fr_FR			ISO-8859-1
fr_FR@euro		ISO-8859-15
is_IS			ISO-8859-1
is_IS.UTF-8		UTF-8
it_IT			ISO-8859-1
ja_JP.eucjp		EUC-JP
se_NO.UTF-8		UTF-8
ta_IN			UTF-8
zh_TW			BIG5
zh_TW			UTF-8
EOF
echo "Generation complete."
 |