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

[alexcrichton]

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.

[alexcrichton]

A related change in wasi-libc, for example, is WebAssembly/wasi-libc#696