posix: provide timespec_to_clock_timeoutms

Provide a private internal function timespec_to_clock_timeoutms() to
complement timespec_to_timeoutms(). This new variant accepts a clock_t
parameter that allows the caller to specify which clock to use.

The original timespec_to_timeoutms() then just becomes a static inline
wrapper around the original.

Note: timespec_to_clock_timeoutms() and timespec_to_timeoutms() might
have a limited lifespan, since it might make sense to create a
common timespec manipulation library.

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