Bluetooth: Increase hci_tx stack size when CONFIG_NO_OPTIMIZATIONS

When optimizations are disabled more RAM is used and we get a stack
overflow on BT_HCI_TX_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>