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: e634d8fc ("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