[3.12] GH-111804: Drop posix.fallocate() under WASI (GH-111869) (GH-111919)

GH-111804: Drop posix.fallocate() under WASI (GH-111869)

Drop posix.fallocate() under WASI.

The underlying POSIX function, posix_fallocate(), was found to vary too
much between implementations to remain in WASI. As such, while it was
available in WASI preview1, it's been dropped in preview2.