remove GCC 4.2.1 build infrastructure

As described in Warner's email message[1] to the FreeBSD-arch mailing
list we have reached GCC 4.2.1's retirement date. At this time all
supported architectures either use in-tree Clang, or rely on external
toolchain (i.e., a contemporary GCC version from ports).

GCC 4.2.1 was released July 18, 2007 and was imported into FreeBSD later
that year, in r171825. GCC has served us well, but version 4.2.1 is
obsolete and not used by default on any architecture in FreeBSD. It
does not support modern C and does not support arm64 or RISC-V.

Thanks to everyone responsible for maintaining, updating, and testing
GCC in the FreeBSD base system over the years.

So long, and thanks for all the fish.

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

PR: 228919
Reviewed by: brooks, imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23124