Inherit Linux semantics for fd_pwrite with O_APPEND

[alexcrichton]

This commit updates the implementation of fd_pwrite in WASI to match Linux semantics for an under-specified corner of WASI. Specifically if fd_pwrite is used the offset specified is ignored if the file is opened in append mode and the bytes are instead appended.

This commit additionally refactors fd_write and fd_pwrite to have basically the same code with only a minor branch internally when the final write is being performed to help deduplicate more logic.

Closes #8817

[fitzgen]

Redirecting to @pchickey

[alexcrichton]

CI turned up an interesting phenomena which makes sense in retrospect. With wasi-common the behavior is subject to whatever the platform OS provides which makes Linux the odd-one-out. Papering over OS differences is easy enough in wasmtime-wasi but is not trivial in wasi-common.

@pchickey how do you feel about sticking with these semantics and just ignoring the whole test in wasi-common? Alternatives could include not landing this in Wasmtime and taking it up in an issue with WASI and/or adding more stuff to wasi-common.

[pchickey]

I think it is fine to just leave this bug in wasi-common and ignore the test - I don't think there is much value in bringing bug fixes for corner cases like this, if users need the fix the real question is why they havent upgraded to wasmtime-wasi.