[bios] Allow bios_console.c to be compiled for x86_64

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/firmware/pcbios/bios_console.c

 					   "pushfw\n\t"
 					   "popw %w0\n\t"
 					   "cli\n\t" )
-			       : "=r" ( flags ), "=a" ( discard_a )
+			       : "=R" ( flags ), "=a" ( discard_a )
 			       : "a" ( 0x1100 ), "m" ( bios_inject_lock ) );
 	bios_inject_lock--;
 	return ( ! ( flags & ZF ) );