tile: support static_key usage in non-module __exit sections

Previously, all the __exit sections were just dropped by the link phase.
However, if there are static_key (jump label) constructs in __exit
sections that are not modules, the link fails with the message:

`.exit.text' referenced in section `__jump_table' of xxx.o:
defined in discarded section `.exit.text' of xxx.o

Support this usage by keeping the .exit.text sections in the final image
if JUMP_LABEL is defined, then discarding them once initialization is
complete.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jason Baron <[e-mail]>
Signed-off-by: Chris Metcalf <[e-mail]>
Cc: "David S. Miller" <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Cc: Heiko Carstens <[e-mail]>
Cc: Joe Perches <[e-mail]>
Cc: Martin Schwidefsky <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/tile/kernel/vmlinux.lds.S

···
   /* "Init" is divided into two areas with very different virtual addresses. */
   INIT_TEXT_SECTION(PAGE_SIZE)
 
+  /*
+   * Some things, like the __jump_table, may contain symbol references
+   * to __exit text, so include such text in the final image if so.
+   * In that case we also override the _einittext from INIT_TEXT_SECTION.
+   */
+#ifdef CONFIG_JUMP_LABEL
+  .exit.text : {
+    EXIT_TEXT
+    _einittext = .;
+  }
+#endif
+
   /* Now we skip back to PAGE_OFFSET for the data. */
   . = (. - TEXT_OFFSET + PAGE_OFFSET);
   #undef LOAD_OFFSET