Consider including a pthread.h for all targets

[dicej]

As part of the WASIp2 work, I changed the Makefile to produce a separate include directory for each target, only adding a pthread.h to the wasm32-wasi-threads one (and its wasm32-wasip1-threads alias). That was a change from the old behavior, where we had a single include directory for all targets, with the result that they all had pthread.h.

Lately, I've been helping with the WASI port of .NET, where pthread.h is used in the runtime library for all TARGET_UNIX platforms. That worked fine for older versions of wasi-sdk, since pthread.h was (accidentally?) available for the wasm32-wasi target, but doesn't work for wasi-sdk 22.

We can certainly change the #ifdef TARGET_UNIX conditionals that guard use of pthread.h to e.g. #if (defined TARGET_UNIX) && !(defined TARGET_WASI), but I'm wondering if it would be useful to include some form of pthread.h in the include directories for all targets in wasi-libc. This would make it easier for projects like .NET to upgrade the wasi-sdk version they're using, as well as make it easier to port other software that uses pthread.h but doesn't necessarily ever create more than one thread.

Thoughts?