UML: define RUNTIME_DISCARD_EXIT

UML: define RUNTIME_DISCARD_EXIT

With CONFIG_VIRTIO_UML=y, GNU ld < 2.36 fails to link UML vmlinux
(w/wo CONFIG_LD_SCRIPT_STATIC).

  `.exit.text' referenced in section `.uml.exitcall.exit' of arch/um/drivers/virtio_uml.o: defined in discarded section `.exit.text' of arch/um/drivers/virtio_uml.o
  collect2: error: ld returned 1 exit status

This fix is similar to the following commits:

- 4b9880dbf3bd ("powerpc/vmlinux.lds: Define RUNTIME_DISCARD_EXIT")
- a494398bde27 ("s390: define RUNTIME_DISCARD_EXIT to fix link error
  with GNU ld < 2.36")
- c1c551bebf92 ("sh: define RUNTIME_DISCARD_EXIT")

Fixes: 99cb0d917ffa ("arch: fix broken BuildID for arm64 and riscv")
Reported-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Richard Weinberger <richard@nod.at>