um: Don't discard .text.exit section

Commit e41f501d3912 ("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: e41f501d3912 ("vmlinux.lds: account for destructor sections")
Reported-by: Stefan Traby <[e-mail]>
Signed-off-by: Andrey Ryabinin <[e-mail]>
Cc: <[e-mail]>
Acked-by: Dmitry Vyukov <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

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

···
   .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 : {