x86: Document linker script ASSERT() quirk

Older binutils breaks if ASSERT() is used without a sink
for the output.

For example 2.14.90.0.6 is known to be broken, the link
fails with:

  LD      .tmp_vmlinux1
  ld:arch/x86/kernel/vmlinux.lds:678: parse error

Document this quirk in all three files that use it.

  See:    http://marc.info/?l=linux-kbuild&m=124930110427870&w=2
  See[2]: d2ba8b21 ("x86: Fix assert syntax in vmlinux.lds.S")

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
LKML-Reference: <4AD6523D.5030909@zytor.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/boot/setup.ld

@@ -53,6 +53,9 @@ SECTIONS
 
 	/DISCARD/ : { *(.note*) }
 
+	/*
+	 * The ASSERT() sink to . is intentional, for binutils 2.14 compatibility:
+	 */
 	. = ASSERT(_end <= 0x8000, "Setup too big!");
 	. = ASSERT(hdr == 0x1f1, "The setup header has the wrong offset!");
 	/* Necessary for the very-old-loader check to work... */

arch/x86/kernel/acpi/realmode/wakeup.lds.S

@@ -57,5 +57,8 @@ SECTIONS
 		*(.note*)
 	}
 
+	/*
+	 * The ASSERT() sink to . is intentional, for binutils 2.14 compatibility:
+	 */
 	. = ASSERT(_end <= WAKEUP_SIZE, "Wakeup too big!");
 }

arch/x86/kernel/vmlinux.lds.S

@@ -305,6 +305,9 @@ SECTIONS
 
 
 #ifdef CONFIG_X86_32
+/*
+ * The ASSERT() sink to . is intentional, for binutils 2.14 compatibility:
+ */
 . = ASSERT((_end - LOAD_OFFSET <= KERNEL_IMAGE_SIZE),
 	   "kernel image bigger than KERNEL_IMAGE_SIZE");
 #else