x86-64: Fix calls via absolute function pointers

linkers don't treat relocations using symindex 0 (undefined)
very well, it can't be misused as indicator for an absolute number.
Just don't bother with special casing this, rather emit an indirect
call/jump right away. ARM64 needs the same (and didn't handle
calls via constant absolute func pointers before).

The testcase as is doesn't fail without the patch, it actually
needs separate compilation (to -fPIC .o file, then to shared lib)
to fail.

tests/tests2/07_function.c

@@ -15,6 +15,11 @@ void qfunc()
    printf("qfunc()\n");
 }
 
+void zfunc()
+{
+   ((void (*)(void))0) ();
+}
+
 int main()
 {
    printf("%d\n", myfunc(3));