retire BSD_CRTBEGIN option

BSD crt is currently used on all architectures (other than sparc64).
Remove the option and use BSD crt everywhere as part of the GCC 4.2.1
retirement plan.

https://lists.freebsd.org/pipermail/freebsd-arch/2020-January/019823.html

PR: 239851
Reviewed by: andrew, brooks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23122