MIPS: Define RUNTIME_DISCARD_EXIT in LD script

MIPS: Define RUNTIME_DISCARD_EXIT in LD script

commit 6dcbd0a69c84a8ae7a442840a8cf6b1379dc8f16 upstream.

MIPS's exit sections are discarded at runtime as well.

Fixes link error:
`.exit.text' referenced in section `__jump_table' of fs/fuse/inode.o:
defined in discarded section `.exit.text' of fs/fuse/inode.o

Fixes: 99cb0d917ffa ("arch: fix broken BuildID for arm64 and riscv")
Reported-by: "kernelci.org bot" <bot@kernelci.org>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>