Fix build failure on vxworks #127084
PR to address issue #127084 .
1. Skip `reset_segpipe` for vxworks
2. Return unimplemented error for vxworks from settimes and lchown
3. Temporarily skip dirfd for vxworks
4. Add allow unused unsafe on read_at and write_at functions in unix/fs.rs
5. Using cfg disable ON_BROKEN_PIPE_FLAG_USED and on_broken_pipe_flag_used() for vxworks
6. Remove old crate::syscommon:🧵:min_stack() reference from process_vxworks.rs and update to set stack size of rtpthread
Thank you.
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