locale.c: Properly return LC_ALL locale names

locale.c: Properly return LC_ALL locale names

On systems using POSIX 2008 functions that don't have querylocale(),
perl has to keep track of the locale names currently in effect.  There
was code to invalidate LC_ALL as a single name if one or more of its
components aren't all the same.  But omitted was code to re-validate it
when they later do become the same.  This commit adds that omitted code.