Trusty: Implement `write_vectored` for stdio
Currently, `write` for stdout and stderr on Trusty is implemented with the semantics of `write_all`. Instead, call the underlying syscall only once in `write` and use the default implementation of `write_all` like other platforms. Also, implement `write_vectored` by adding support for `IoSlice`.
Refactor stdin to reuse the unsupported type like https://github.com/rust-lang/rust/pull/136769.
It requires #138875 to fix the build for Trusty, though they do not conflict and can merge in either order.
cc `@randomPoison`
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