Use prebuilt m4 for bison too

Bison is currently looking for m4 directly in `/usr/bin/m4`, instead of
through $PATH, so it hadn't been tracked via the $PATH interposer
checks. But if you don't install m4 at all, it starts failing.

Bug: 117561006
Bug: 130111713
Test: Run on a machine without m4, build passes
Change-Id: I3fbad64127c183de79c17511e979fd5df946eb80

cc/gen.go

@@ -98,6 +98,7 @@ func genYacc(ctx android.ModuleContext, rule *android.RuleBuilder, yaccFile andr
 	}
 
 	cmd.Text("BISON_PKGDATADIR=prebuilts/build-tools/common/bison").
+		FlagWithInput("M4=", ctx.Config().PrebuiltBuildTool(ctx, "m4")).
 		Tool(ctx.Config().PrebuiltBuildTool(ctx, "bison")).
 		Flag("-d").
 		Flags(flags).