riscv: lds: define RUNTIME_DISCARD_EXIT

riscv: lds: define RUNTIME_DISCARD_EXIT

riscv discards .exit.* sections at run-time but doesn't define
RUNTIME_DISCARD_EXIT. However, the .exit.* sections are still allocated
and kept even if the generic DISCARDS would discard the sections due
to missing RUNTIME_DISCARD_EXIT, because the DISCARD sits at the end of
the linker script. Add the missing RUNTIME_DISCARD_EXIT define so that
it still works if we move DISCARD up or even at the beginning of the
linker script.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Suggested-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230119155417.2600-2-jszhang@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>