Use /proc/self/cmdline on Solaris as well in shell_global_reexec_self()

Solaris 11.3.5 and later have a Linux-compatible implementation of
/proc/self/cmdline, so the code to use it can be enabled in gnome-shell
on Solaris - if used on an older OS, it simply fails to open the file
and returns without doing anything, just as the code did before enabling
this.

https://bugzilla.gnome.org/show_bug.cgi?id=776199

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>

src/shell-global.c

@@ -1258,7 +1258,7 @@ shell_global_reexec_self (ShellGlobal *global)
   GPtrArray *arr;
   gsize len;
 
-#if defined __linux__
+#if defined __linux__ || defined __sun
   char *buf;
   char *buf_p;
   char *buf_end;