Auto merge of #136780 - joboet:move_pal_stdio, r=Amanieu

std: move stdio to `sys`

As per #117276, this moves the platform definitions of `Stdout` and friends into `sys`. This PR also unifies the UNIX and Hermit implementations and moves the `__rust_print_err` function needed by libunwind on SGX into the dedicated module for such helper functions.