Allow build of linuxboot.S with old assemblers

In the spirit of ff56954b, fix the
build of linuxboot.S with old as(1) (as found in some BSD base systems)
by emitting the bytes of the insn it doesn't like instead.

Signed-off-by: Juergen Lock <nox@jelal.kn-bremen.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

pc-bios/optionrom/optionrom.h

@@ -66,7 +66,10 @@
 	outw		%ax, (%dx);			\
 	mov		$BIOS_CFG_IOPORT_DATA, %dx;	\
 	cld;						\
-	rep insb	(%dx), %es:(%edi);
+	/* old as(1) doesn't like this insn so emit the bytes instead: \
+	rep insb	(%dx), %es:(%edi);		\
+	*/						\
+	.dc.b		0x67,0xf3,0x6c
 
 #define OPTION_ROM_START					\
     .code16;						\