x86: correct/clarify comment in nops.h

<asm-x86/nops.h> describes certain multibyte instructions as
"generic" nops when in fact they aren't nops at all in 64-bit
mode (missing REX.W causing truncation of a register).

Update the comment to state that K8 or P6 style nops should be
used in 64-bit mode. This matches what the alternatives code does.
Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

include/asm-x86/nops.h

@@ -5,6 +5,8 @@
 
 /* generic versions from gas
    1: nop
+   the following instructions are NOT nops in 64-bit mode,
+   for 64-bit mode use K8 or P6 nops instead
    2: movl %esi,%esi
    3: leal 0x00(%esi),%esi
    4: leal 0x00(,%esi,1),%esi