tests: kernel: threads_customdata: increase STACK_SIZE to 512 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 increase stack size to 512 for
threads_customdata, otherwise we experience stack overflow.

Change-Id: I805bc346b8a2c2f4ad6d0db622eb262290af942b
Signed-off-by: Jean-Paul Etienne <fractalclone@gmail.com>