objtool/klp: Fix vmlinux .klp.symid link error for .exitcall.exit symbols

Building a kernel via klp-build fails to link:

  `__exitcall_aes_mod_exit' referenced in section `.klp.symid' of vmlinux.o: defined in discarded section `.exitcall.exit' of vmlinux.o
  `__exitcall_dax_exit' referenced in section `.klp.symid' of vmlinux.o: defined in discarded section `.exitcall.exit' of vmlinux.o
  `__exitcall_hid_exit' referenced in section `.klp.symid' of vmlinux.o: defined in discarded section `.exitcall.exit' of vmlinux.o
  `__exitcall_usb_serial_module_exit' referenced in section `.klp.symid' of vmlinux.o: defined in discarded section `.exitcall.exit' of vmlinux.o

module_exit() on a built-in emits a static __exitcall_$fn pointer into
.exitcall.exit, which vmlinux.lds.h discards unconditionally via
EXIT_CALL.  When two built-in translation units define a module_exit()
function of the same name, the resulting local symbols collide,
symid_needed() sees a duplicate and emits a .klp.symid entry for each,
referencing symbols the linker then throws away.

Same-named module_exit() functions are not rare:

  dax_exit                drivers/dax/device.c, drivers/dax/fsdev.c
  hid_exit                drivers/hid/hid-core.c, drivers/hid/usbhid/hid-core.c
  aes_mod_exit            arch/arm64/crypto/aes-ce-ccm-glue.c, lib/crypto/aes.c
  usb_serial_module_exit  module_usb_serial_driver() expands to this fixed
                          name in each of its ~49 users

The last one makes the collision structural rather than accidental: any
kernel with two built-in USB serial drivers has it.  This is not arch
specific either; it only requires the objects to be built in rather than
modular, which is why a monolithic config trips it while a typical
distro config does not.

Add .exitcall.exit to the discarded section list so its symbols don't get
symids.

This is the same failure mode as "objtool/klp: Fix vmlinux .klp.symid
link error for .no_trim_symbol symbols", for another unconditionally
discarded allocated section.

Fixes: 029223d301 ("objtool/klp: Add .klp.symid for sympos disambiguation")
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://patch.msgid.link/m24igzlbxf.fsf@kernel.org
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>

tools/objtool/klp-symid.c

@@ -30,6 +30,7 @@
 
 static const char * const discarded_secs[] = {
 	".discard",
+	".exitcall.exit",
 	".modinfo",
 	".no_trim_symbol",
 	"__tracepoint_check",