Fix prctl argument type

Although prctl is declared as a varargs function, in actuality it takes
unsigned long arguments (after the first int argument). This patch
ensures that the slack value passed to prctl is correctly sized.

Without this change, the 32-bit ARM compiler places the 64-bit slack
value in registers r2 and r3, but prctl expects the first argument to be
in r1. For kernel versions < 4.6, set_sched_policy has not been
correctly setting the timer slack value in 32-bit ARM code.

Test: Run on bullhead device. Verified assembly of libcutils.so

Change-Id: Ie0e22cbf74a74ff168b257b2e58d0c252449d6c9