crunchgen: fix "keep" for an ELF world, break it out

"keep" currently adds a leading underscore, which hasn't been useful or
accurate since a.out days. Preserve the symbol name as it's given
rather than mangle it to match ELF-style symbol names.

This was partially fixed back in
6cd35234a092d ("Assume ELF-style symbol names now.") for crunchgen, but
the keeplist wasn't changed to match it.

While we're here, break it out to bsd.crunchgen.mk for later use in
bsdbox.

Reviewed by: adrian, imp
Differential Revision: https://reviews.freebsd.org/D42499