cgo: only include the symbols that are necessary (recursively)

Only try to convert the C symbols to their Go equivalents that are
actually referenced by the Go code with C.<somesymbol>. This avoids
having to support all possible C types, which is difficult because of
oddities like `typedef void` or `__builtin_va_list`. Especially
__builtin_va_list, which varies between targets.

testdata/cgo/main.h

@@ -2,12 +2,17 @@
 #include <stdint.h>
 
 typedef short myint;
+typedef short unusedTypedef;
 int add(int a, int b);
+int unusedFunction(void);
 typedef int (*binop_t) (int, int);
 int doCallback(int a, int b, binop_t cb);
 typedef int * intPointer;
 void store(int value, int *ptr);
 
+// this signature should not be included by CGo
+void unusedFunction2(int x, __builtin_va_list args);
+
 typedef struct collection {
 	short         s;
 	long          l;
@@ -37,6 +42,7 @@ void unionSetData(short f0, short f1, short f2);
 
 // test globals and datatypes
 extern int global;
+extern int unusedGlobal;
 extern bool globalBool;
 extern bool globalBool2;
 extern float globalFloat;