kbuild: merge vmlinux_link() between ARCH=um and other architectures

For ARCH=um, ${CC} is used as the linker driver. Hence, the linker
options are prefixed with -Wl, .

Merge the similar code.

I replaced the -T option with the long option --script= so that it
works well with/without ${wl}.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>