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

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

src/arch/i386/image/bzimage.c

 					   "pushw %w2\n\t"
 					   "pushw $0\n\t"
 					   "lret\n\t" )
-			       : : "r" ( bzimg.rm_kernel_seg ),
-			           "r" ( bzimg.rm_heap ),
-			           "r" ( bzimg.rm_kernel_seg + 0x20 ) );
+			       : : "R" ( bzimg.rm_kernel_seg ),
+			           "R" ( bzimg.rm_heap ),
+			           "R" ( bzimg.rm_kernel_seg + 0x20 ) );
 
 	/* There is no way for the image to return, since we provide
 	 * no return address.