x86/boot: Add back some padding for the CRC-32 checksum

Even though no uses of the bzImage CRC-32 checksum are known, ensure
that the last 4 bytes of the image are unused zero bytes, so that the
checksum can be generated post-build if needed.

[ mingo: Added the 'obsolete' qualifier to the comment. ]

Suggested-by: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Ian Campbell <ijc@hellion.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20250312081204.521411-2-ardb+git@google.com

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

@@ -48,7 +48,8 @@ SECTIONS
 		*(.data)
 		*(.data.*)
 
-		. = ALIGN(0x200);
+		/* Add 4 bytes of extra space for the obsolete CRC-32 checksum */
+		. = ALIGN(. + 4, 0x200);
 		_edata = . ;
 	}
 	. = ALIGN(L1_CACHE_BYTES);