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() {

cgo/testdata/symbols.out.go

@@ -55,5 +55,10 @@ func C.variadic2(x C.int, y C.int)
 
 var C.variadic2$funcaddr unsafe.Pointer
 
+//export _Cgo_static_173c95a79b6df1980521_staticfunc
+func C.staticfunc!symbols.go(x C.int)
+
+var C.staticfunc!symbols.go$funcaddr unsafe.Pointer
+
 //go:extern someValue
 var C.someValue C.int