x86-64: merge the standard and compat start_thread() functions

The only thing left that differs between the standard and compat
start_thread functions is the actual segment numbers and the
prototype, so have a single common function which contains the guts
and two very small wrappers.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Suresh Siddha <suresh.b.siddha@intel.com>