vdso: replace current_thread_info when building vDSO rather than diking it out

vdso: replace current_thread_info when building vDSO rather than diking it out

Orabug: 20861959

When building the userspace parts of the vDSO code, we have to dike out things
from the various kernel headers we use that generate register relocations,
since we cannot handle relocations in the vDSO.  The principal such thing is
current_thread_info(), which we used to dike out entirely -- but in the -rt
patchset, a lot of things in the headers reference this.  So, instead,
simply have current_thread_info() generate nonsense code that doesn't emit
a relocation, so that its users still compile (though they would never
work -- but that's not important, since they are never used).

Signed-off-by: Nick Alcock <nick.alcock@oracle.com>
Signed-off-by: Allen Pais <allen.pais@oracle.com>
(cherry picked from commit 4f9b3c6e7fe105ea04f89794007f97b43a63c897)