gc: Better error when computing remainder of non-int

Previously the compiler would just emit "internal compiler error" when
trying to compute the remainder of floats or complex types.

R=rsc
CC=golang-dev
https://golang.org/cl/1243041

src/cmd/gc/align.c

@@ -491,7 +491,7 @@ typeinit(void)
 	okfor[OGT] = okforcmp;
 	okfor[OLE] = okforcmp;
 	okfor[OLT] = okforcmp;
-	okfor[OMOD] = okforarith;
+	okfor[OMOD] = okforand;
 	okfor[OMUL] = okforarith;
 	okfor[ONE] = okforeq;
 	okfor[OOR] = okforand;