tests: libcxx: increase ZTEST_STACKSIZE and added platforms for testing

Increasing ZTEST_STACKSIZE to 4096.
This ensures that the riscv32 platforms can succesfully execute the
C++ exception test cases.

Also add the following platforms to allow list:
- qemu_arc_em
- qemu_arc_hs
- qemu_cortex_m0
- qemu_cortex_m3
- qemu_cortex_r5
- qemu_riscv32
- qemu_x86
- qemu_xtensa

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>