commit 4229fb12 upstream.

Userspace return code may skip restoring THREADPTR register if there are
no registers that need to be zeroed. This leads to spurious failures in
libc NPTL tests.

Always restore THREADPTR on return to userspace.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>