x86/process: Re-export start_thread()

The consolidation of the start_thread() functions removed the export
unintentionally. This breaks binfmt handlers built as a module.

Add it back.

Fixes: e634d8fc792c ("x86-64: merge the standard and compat start_thread() functions")
Signed-off-by: Rian Hunter <rian@alum.mit.edu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: Dmitry Safonov <dima@arista.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180819230854.7275-1-rian@alum.mit.edu

arch/x86/kernel/process_64.c

@@ -384,6 +384,7 @@ start_thread(struct pt_regs *regs, unsigned long new_ip, unsigned long new_sp)
 	start_thread_common(regs, new_ip, new_sp,
 			    __USER_CS, __USER_DS, 0);
 }
+EXPORT_SYMBOL_GPL(start_thread);
 
 #ifdef CONFIG_COMPAT
 void compat_start_thread(struct pt_regs *regs, u32 new_ip, u32 new_sp)