Retire legacy target names.

[jaykrell]

Years ago target names become fairly regular:
{I386,AMD64,PPC,PPC64,ALPHA,PA{32,64},ARM[64]}_{LINUX,NT,FREEBSD,OPENBSD,NETBSD,DARWIN}

A few legacy names remain for hypothetically supported targets (is anyone still using 32bit x86?):
LINUXLIBC6 aka I386_LINUX
FreeBSD4 aka I386_FREEBSD
NetBSD2_i386 aka I386_NETBSD

While I'd still like to eliminate the notion of target, I think it is long past time to remove these.
They don't cost much, but still I'd like to clean things up.