x86/boot: Discard .discard.unreachable for arch/x86/boot/compressed/vmlinux

With commit

  ce5e3f9 ("efi/printf: Add 64-bit and 8-bit integer support")

arch/x86/boot/compressed/vmlinux may have an undesired .discard.unreachable
section coming from drivers/firmware/efi/libstub/vsprintf.stub.o. That section
gets generated from unreachable() annotations when CONFIG_STACK_VALIDATION is
enabled.

.discard.unreachable contains an R_X86_64_PC32 relocation which will be
warned about by LLD: a non-SHF_ALLOC section (.discard.unreachable) is
not part of the memory image, thus conceptually the distance between a
non-SHF_ALLOC and a SHF_ALLOC is not a constant which can be resolved at
link time:

  % ld.lld -m elf_x86_64 -T arch/x86/boot/compressed/vmlinux.lds ... -o arch/x86/boot/compressed/vmlinux
  ld.lld: warning: vsprintf.c:(.discard.unreachable+0x0): has non-ABS relocation R_X86_64_PC32 against symbol ''

Reuse the DISCARDS macro which includes .discard.* to drop
.discard.unreachable.

 [ bp: Massage and complete the commit message. ]

Reported-by: kbuild test robot <[email protected]>
Signed-off-by: Fangrui Song <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Tested-by: Arvind Sankar <[email protected]>
Tested-by: Sedat Dilek <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: MaskRay

arch/x86/boot/compressed/vmlinux.lds.S

@@ -73,4 +73,6 @@ SECTIONS
 #endif
 	. = ALIGN(PAGE_SIZE);	/* keep ZO size page aligned */
 	_end = .;
+
+	DISCARDS
 }