ice40: Make HeAP the default placer

Signed-off-by: David Shah <dave@ds0.me>

ice40/arch.cc

diff --git a/ice40/arch.cc b/ice40/arch.cc
index cc50cb6..0f8e596 100644
--- a/ice40/arch.cc
+++ b/ice40/arch.cc
@@ -1245,7 +1245,11 @@
     }
 }
 
+#ifdef WITH_HEAP
+const std::string Arch::defaultPlacer = "heap";
+#else
 const std::string Arch::defaultPlacer = "sa";
+#endif
 
 const std::vector<std::string> Arch::availablePlacers = {"sa",
 #ifdef WITH_HEAP