Add support for the [Zephyr Usermode](https://docs.zephyrproject.org/latest/kernel/usermode/index.html)
to the Zephyr port.
The following changes are applied:
- Fix `signbit`, check if it is defined already and only implement it, if not
- Introduce `sys_mutex` and `sys_sem` in favour of `k_mutex` and `k_sem`, when `CONFIG_USERMODE` is enabled
- Remove the installation of the `_stdout_hook_iwasm()` when `CONFIG_USERMODE` is enabled, otherwise this
causes MPU errors since the std hook is in the kernel space
- Add a thread name for debugging
4c5f35b0e3