These were left over from $FreeBSD$ removal.
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D42612
(cherry picked from commit 1ca63a8219b88b752b064d19bd3428c61dbcf1f9)
this is used by some 3rd party applications when {e,f,g}cvt() are
not found. POSIX defines the xcvt() funtions but says they are
deprecated in favor or sprintf(). We'll import these functions
from OpenBSD and remove __gdtoa() from the exported interfaces
when libc version is bumped.