Fix the signature of the IndexOf entry point.

The IndexOf entry point was declared as taking four
arguments (void*, uint32_t, uint32_t, uint32_t) whereas all
actual implementations use three arguments (void*, uint32_t,
uint32_t).  As that fourth argument is not documented, drop
it from the intrinsic declaration to have it match the
implementations.

Change-Id: I65d747033192025ccd2b9a5e8f8ed05b77a21941