compiler: always use fat function pointers with context

This reduces complexity in the compiler without affecting binary sizes
too much.

Cortex-M0:   no changes
Linux x64:   no changes
WebAssembly: some testcases (calls, coroutines, map) are slightly bigger

compiler/calls.go

@@ -21,6 +21,9 @@ func (c *Compiler) createRuntimeCall(fnName string, args []llvm.Value, name stri
 		panic("trying to call runtime." + fnName)
 	}
 	fn := c.ir.GetFunction(member.(*ssa.Function))
+	if !fn.IsExported() {
+		args = append(args, llvm.Undef(c.i8ptrType)) // unused context parameter
+	}
 	return c.createCall(fn.LLVMFn, args, name)
 }