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 #136769.
Use `std::mem::{size_of, size_of_val, align_of, align_of_val}` from the
prelude instead of importing or qualifying them.
These functions were added to all preludes in Rust 1.80.
