um: Don't discard .text.exit section

Commit e41f501d39 ("vmlinux.lds: account for destructor sections")
added '.text.exit' to EXIT_TEXT which is discarded at link time by default.
This breaks compilation of UML:
     `.text.exit' referenced in section `.fini_array' of
     /usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/libc.a(sdlerror.o):
     defined in discarded section `.text.exit' of
     /usr/lib/gcc/x86_64-linux-gnu/6/../../../x86_64-linux-gnu/libc.a(sdlerror.o)

Apparently UML doesn't want to discard exit text, so let's place all EXIT_TEXT
sections in .exit.text.

Fixes: e41f501d39 ("vmlinux.lds: account for destructor sections")
Reported-by: Stefan Traby <stefan@hello-penguin.com>
Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: <stable@vger.kernel.org>
Acked-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/asm/common.lds.S

@@ -81,7 +81,7 @@
   .altinstr_replacement : { *(.altinstr_replacement) }
   /* .exit.text is discard at runtime, not link time, to deal with references
      from .altinstructions and .eh_frame */
-  .exit.text : { *(.exit.text) }
+  .exit.text : { EXIT_TEXT }
   .exit.data : { *(.exit.data) }
 
   .preinit_array : {