allow ports to override go behavior

Split the arch-specific logic out of the common go code and into a dedicated
weak function called do_go_exec() that lives in cpu directories.  This will
need review from i386/nios people to make sure I didn't break them.
diff --git a/lib_i386/board.c b/lib_i386/board.c
index 47fbab4..22191e6 100644
--- a/lib_i386/board.c
+++ b/lib_i386/board.c
@@ -421,3 +421,11 @@
 	puts ("### ERROR ### Please RESET the board ###\n");
 	for (;;);
 }
+
+unsigned long do_go_exec (ulong (*entry)(int, char *[]), int argc, char *argv[])
+{
+	/*
+	 * Nios function pointers are address >> 1
+	 */
+	return (entry >> 1) (argc, argv);
+}