synth_ice40: use 4-LUT coarse synthesis mode.

techlibs/ice40/synth_ice40.cc

diff --git a/techlibs/ice40/synth_ice40.cc b/techlibs/ice40/synth_ice40.cc
index c2aed87..4c4fdf7 100644
--- a/techlibs/ice40/synth_ice40.cc
+++ b/techlibs/ice40/synth_ice40.cc
@@ -226,7 +226,7 @@
 
 		if (check_label("coarse"))
 		{
-			run("synth -run coarse");
+			run("synth -lut 4 -run coarse");
 		}
 
 		if (!nobram && check_label("bram", "(skip if -nobram)"))