Add W_STOPCODE to construct a stop code
Add W_STOPCODE to construct a stop code

On Linux, `sys/wait.h` defines a `W_STOPCODE` macro to construct a stop
code from the signal number of a stopping signal. Provide an equivalent
function.

Suggested-by: Ivan Tham <1caaae66ad2b6814584001fdaa761fdc4f6b3dff@riseup.net>

rust-lang/libc,rust-lang/libc,rust-lang/libc