Simplify the generation of ld scripts for Linux and Solaris

Because we know for certain that the link_shlib targets are used
exclusively for shared libraries (libcrypto and libssl) and that they
must have an associated .num file, we don't need to check the library
name to produce an ld script.  Just do it unconditionally.

link_shlib.linux-shared can be simplified further, as most of it is
exactly the same as $(DO_GNU_SO) with just one variable modification.

Reviewed-by: Andy Polyakov <appro@openssl.org>