libmain: ensure initfn is called when loading a go library

AIX does not support .init_array.
The alterative is to export the __go_init function and tell the linker
it is an init function with the -Wl,-binitfini:__go_init option.

Issue golang/go#19200

Change-Id: I4ef52a17e646f80bd37d141483ec3086c87fcf30
Reviewed-on: https://go-review.googlesource.com/37965
Reviewed-by: Ian Lance Taylor <iant@golang.org>

Author: MatthieuSarter

libgo/runtime/go-libmain.c

@@ -24,7 +24,12 @@
    and calls exported Go functions as needed.  */
 
 static void die (const char *, int);
-static void initfn (int, char **, char **);
+/* .init_array section does not exist in AIX XCOFF.
+   -Wl,-binitfini:__go_init option will be required to build go
+   libraries and make sure __go_init is called when the library is
+   loaded. This requires __go_init to be exported.  */
+
+void __go_init (int, char **, char **);
 static void *gostart (void *);
 
 /* Used to pass arguments to the thread that runs the Go startup.  */
@@ -34,6 +39,7 @@ struct args {
   char **argv;
 };
 
+#ifndef _AIX
 /* We use .init_array so that we can get the command line arguments.
    This obviously assumes .init_array support; different systems may
    require other approaches.  */
@@ -42,16 +48,17 @@ typedef void (*initarrayfn) (int, char **, char **);
 
 static initarrayfn initarray[1]
 __attribute__ ((section (".init_array"), used)) =
-  { initfn };
+  { __go_init };
+#endif
 
 /* This function is called at program startup time.  It starts a new
    thread to do the actual Go startup, so that program startup is not
    paused waiting for the Go initialization functions.  Exported cgo
    functions will wait for initialization to complete if
    necessary.  */
 
-static void
-initfn (int argc, char **argv, char** env __attribute__ ((unused)))
+void
+__go_init (int argc, char **argv, char** env __attribute__ ((unused)))
 {
   int err;
   pthread_attr_t attr;