tests: kernel: test_mpool_concept: increase STACK_SIZE to 1024 for riscv32

In RISCV, stack always grows by a multiple of 16 bytes, even if we are saving
data of size < 16 bytes onto the stack.

Hence, for riscv32 architecture a bigger stack size is required for
test_mpool_concept, otherwise we experience stack overflow.

Change-Id: I938aa511efcae66f0131fa1bc23bd68600421885
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>