libs: Avoid exposing -Wl,--version-script to other built library

libs: Avoid exposing -Wl,--version-script to other built library

$(SHLIB_LDFLAGS) is used by more targets that the single targets that
except it (libxenfoo.so.X.Y). There is also some dynamic libraries in
stats/ that uses $(SHLIB_LDFLAGS) (even if those are never built), and
there's libxenlight_test.so which doesn't needs a version script.

Also, libxenlight_test.so might failed to build if the version script
doesn't exist yet.

For these reasons, avoid changing the generic $(SHLIB_LDFLAGS) flags,
and add the flag directly on the command line.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Henry Wang <Henry.Wang@arm.com>