kernel: Make thread 'init_delay' k_timeout_t rather than int msecs

Storing this value in milliseconds rather than using k_timeout_t requires
the system to perform division at runtime to convert types. This pulls in
the 64-bit soft division code on platforms without hardware for this.

Perform the conversion at build time instead by using the runtime time
directly.

The init_delay field was moved within the _static_thread_data structure to
avoid introducing a hole for alignment on 32-bit systems when using 64-bit
timeouts.

Use SYS_TIMEOUT_MS instead of K_MSEC so that the initial delay can be set
to forever.

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