Separate general linking flags from extra libraries

So far, we've placed all extra library related flags together, ending
up in the make variable EX_LIBS.  This turns out to be problematic, as
for example, some compilers don't quite agree with something like
this:

    cc -o foo foo.o -L/whatever -lsomething

They prefer this:

    cc -L/whatever -o foo foo.o -lsomething

IBM's compiler on OS/390 is such a compiler that we know of, and we
have previously handled that as a previous case.

The answer here is to make a more general solution, where linking
options are divided in two parts, where one ends up in LDFLAGS and
the other in EX_LIBS (they corresponds to what is called LDFLAGS and
LDLIBS in the GNU world)

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5033)