sys: timeutil: add utility functions for struct timespec

Add a number of utility functions for manipulating struct timespec.

* timespec_add()
* timespec_compare()
* timespec_equal()
* timespec_is_valid()
* timespec_negate()
* timespec_normalize()
* timespec_sub()
* timespec_from_timeout()
* timespec_to_timeout()

If the `__builtin_add_overflow()` function is available, then the
API is mostly branchless, which should provide decent performance on
systems with an instruction cache and branch prediction. Otherwise,
manually written alternatives exist that are also perhaps more
readable.

The two functions at the end convert time durations between
representation as `struct timespec` and `k_timeout_t`.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>