arm64: vmlinux.lds.S: don't discard .exit.* sections at link-time

.exit.* sections may be subject to patching by the new alternatives
framework and so shouldn't be discarded at link-time. Without this patch,
such a section will result in the following linker error:

`.exit.text' referenced in section `.altinstructions' of
 drivers/built-in.o: defined in discarded section `.exit.text' of
drivers/built-in.o

Signed-off-by: Will Deacon <will.deacon@arm.com>