[3.12] GH-115978: Disable `*readv()` and `*writev()` on WASI (GH-116228) (GH-116231)

GH-115978: Disable `*readv()` and `*writev()` on WASI (GH-116228)

Wasmtime doesn't implement these functions in a way to pass test_posix (https://github.com/bytecodealliance/wasmtime/issues/7830).
(cherry picked from commit 5dc8c84d397110f9edfa56793ad8887b1f176d79)

Co-authored-by: Brett Cannon <brett@python.org>