valgrind: skip the boost_thread test on arm

This test is failing on the arm workers only so skip there until the issue
can be worked on and resolved. The bug #14311 will remain open for tracking.

Signed-off-by: Randy MacLeod <Randy.MacLeod@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit d98deec9)
Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>