libc: use separate collate objects for C, POSIX, and C.UTF-8

Fix newlocale() overwriting the locale name in collate object
when same instance was used for those locales, and querylocale()
reporting unexpected value for LC_COLLATE_MASK.

PR: 255646, 269375
Reviewed by: markj, bapt (previous version)
Differential Revision: https://reviews.freebsd.org/D30146