Optimize libfp. Don't lose -O6 in ackcflags.

This drops 124 bytes from the mandelbrot command (from 15015 to 14891
bytes) but has almost no effect on performance; the command takes
about 144 seconds (in YAZE-AG) both before and after optimizing libfp.

mach/proto/fp/build.lua

@@ -19,7 +19,7 @@ for _, plat in ipairs({"cpm"}) do
 			},
 			suffix = ".s",
 			vars = {
-				ackcflags = "-I"..dirname(byte_order),
+				["+ackcflags"] = { "-I"..dirname(byte_order) },
 				plat = plat
 			}
 		}