subsys/logging: Increase logging stack size for assert or spin_validate

These two options significantly increase the amount of stack space used
by the logging thread as these generate both additional logging and
generate deep stacks themselves. This fixes running the
tests/subsys/logging/log_core_additional/logging.add.log1 test on
riscv32 with THREAD_LOCAL_STORAGE enabled.

Signed-off-by: Keith Packard <keithp@keithp.com>