makesyscalls: generate private syscall symbols

For libsys we need to expose all the private symbols (_ and __sys_
prefixes) so libsys can replace the libc versions. Rather than trying
to maintain a table, teach makesyscalls to generate it.

There are a small number of "_" prefixed symbols that are exposed as
public interfaces rather than in the private symbol space. Since the
list is short, just hardcode it for now.

If doesn't appear that we need to export freebsd#_foo symbols for compat
system calls explicitly. If it turns out we do, there are probably few
enough of them to handle seperately.

Reviewed by: kib, emaste, imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/908