cgo: implement support for static functions

cgo/testdata/symbols.go

@@ -5,6 +5,7 @@ package main
 int foo(int a, int b);
 void variadic0();
 void variadic2(int x, int y, ...);
+static void staticfunc(int x);
 
 // Global variable signatures.
 extern int someValue;
@@ -16,6 +17,7 @@ func accessFunctions() {
 	C.foo(3, 4)
 	C.variadic0()
 	C.variadic2(3, 5)
+	C.staticfunc(3)
 }
 
 func accessGlobals() {