MIPS: Define RUNTIME_DISCARD_EXIT in LD script

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: 99cb0d91 ("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>

arch/mips/kernel/vmlinux.lds.S

@@ -15,6 +15,8 @@
 #define EMITS_PT_NOTE
 #endif
 
+#define RUNTIME_DISCARD_EXIT
+
 #include <asm-generic/vmlinux.lds.h>
 
 #undef mips