prctl: reset PR_SET_TIMERSLACK_PID value to avoid conflict

PR_SET_TIMERSLACK_PID value keep colliding with that of
newer prctls in mainline (e.g. first with PR_SET_THP_DISABLE,
and again with PR_MPX_ENABLE_MANAGEMENT).

So reset PR_SET_TIMERSLACK_PID to a large number so as to
avoid conflict in the near term while it is out of mainline
tree.

Corresponding Change-Id up for review in platform/system/core
is Icd8c658c8eb62136dc26c2c4c94f7782e9827cdb

Change-Id: I061b25473acc020c13ee22ecfb32336bc358e76a
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>