Separate general linking flags from extra libraries

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)