logging: Increase log processing stack size when NO_OPTIMIZATIONS

When optimizations are disabled more RAM is used and we get a stack
overflow on CONFIG_LOG_PROCESS_THREAD_STACK_SIZE. To rectify this,
increase the stack size when CONFIG_NO_OPTIMIZATIONS.

This does not scale well, and will have to be replaced by a a more
general solution eventually, but in the mean time it follows the
existing best practice established by the GCOV infrastructure in
commit e908ea9a

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>