[libc] Remove `MAX_LANE_SIZE` definition from the RPC server

This `MAX_LANE_SIZE` was a hack from the days when we used a single
instance of the server and had some GPU state handle it. Now that we
have everything templated this really shouldn't be used. This patch
removes its use and replaces it with template arguments.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D158633