cgo: implement support for static functions

testdata/cgo/extra.go

@@ -3,4 +3,18 @@ package main
 // Make sure CGo supports multiple files.
 
 // int fortytwo(void);
+// static float headerfunc_static(float a) { return a - 1; }
+// static void headerfunc_void(int a, int *ptr) { *ptr = a; }
 import "C"
+
+func headerfunc_2() {
+	// Call headerfunc_static that is different from the headerfunc_static in
+	// the main.go file.
+	// The upstream CGo implementation does not handle this case correctly.
+	println("static headerfunc 2:", C.headerfunc_static(5))
+
+	// Test function without return value.
+	var n C.int
+	C.headerfunc_void(3, &n)
+	println("static headerfunc void:", n)
+}

testdata/cgo/main.go

@@ -12,6 +12,7 @@ int mul(int, int);
 import "C"
 
 // int headerfunc(int a) { return a + 1; }
+// static int headerfunc_static(int a) { return a - 1; }
 import "C"
 
 import "unsafe"
@@ -47,6 +48,8 @@ func main() {
 
 	// functions in the header C snippet
 	println("headerfunc:", C.headerfunc(5))
+	println("static headerfunc:", C.headerfunc_static(5))
+	headerfunc_2()
 
 	// equivalent types
 	var goInt8 int8 = 5

testdata/cgo/out.txt

@@ -16,6 +16,9 @@ callback 2: 600
 variadic0: 1
 variadic2: 15
 headerfunc: 6
+static headerfunc: 4
+static headerfunc 2: +4.000000e+000
+static headerfunc void: 3
 bool: true true
 float: +3.100000e+000
 double: +3.200000e+000