clocks: Cannot implement `clock_nanosleep(CLOCK_REALTIME, ...)` in C

Currently WASIp{2,3} WITs for wasi:clocks only support subscribing to the monotonic clock, not the wall clock. This means that it's required for C programs to sleep with CLOCK_MONOTONIC instead of CLOCK_REALTIME. This issue was papered over with the historical WASIp1-to-WASIp2 adapter and never surfaced, but the libc implementation is slightly different in that it rejects sleeping with CLOCK_REALTIME since that's not actually supported. I'm not aware of a reason to specifically not support this, so I wanted to file an issue to eventually add the same-style subscribe* functions to the wasi:clocks/wall-clock interface.

Start by reading the wasi:clocks WITs, especially the wasi:clocks/wall-clock interface, and compare them with the monotonic-clock subscribe* functions. Trace the libc handling of clock_nanosleep and determine the interface changes needed so CLOCK_REALTIME is supported; done means the wall-clock subscription path and corresponding C behavior are implemented and verified.