crunchgen(8): fix crunched application build with WARNS=6

This was revealed by the rescue build with a patch I'm working on to default
WARNS=6 everywhere. The issues resolved were:

• Missing prototype for _crunched_${ident}_stub in the *_stub.c generated bits
• Missing prototype for crunched_main
• Incomplete prototype for _crunched_${ident}_stub in the generated parts of crunched_main
• Literal strings in the stub table must drop const qualifier, unless we const'ify name
• f field in struct stub didn't have a proper prototype

Most of these issues are minor formalities and easily addressed.

I note that if my patch to eventually raise WARNS for the rescue build
lands, we'll need to bump the __FreeBSD_version requirement for
bootstrapping crunchgen and wipe out the rescue .OBJDIR if it's stale, which
we should be able to detect pretty easily from a couple of the issues that
have been fixed here.

Reviewed by: arichardson
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26363