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: NWill Deacon <will.deacon@arm.com>

arch/arm64/kernel/vmlinux.lds.S

@@ -11,8 +11,9 @@
 
 #include "image.h"
 
-#define ARM_EXIT_KEEP(x)
-#define ARM_EXIT_DISCARD(x)	x
+/* .exit.text needed in case of alternative patching */
+#define ARM_EXIT_KEEP(x)	x
+#define ARM_EXIT_DISCARD(x)
 
 OUTPUT_ARCH(aarch64)
 ENTRY(_text)