benchmark/latency: reduce RAM requirements

Reduce RAM requirements of latency benchmarks by reducing the amount of
memory statically allocated for stacks (both fiber and task). This helps
reduce the memory usage required by microkernel test to below 20kB,
enabling the benchmark to be run on Nucleo-F103RB board. The change was
verified for nucleo_f103rb and qemu_x86 boards.

Change-Id: If5990d107f0fcfabd12ade82f97e7a1a13f8421c
Signed-off-by: Maciej Borzecki <maciek.borzecki@gmail.com>