libc 0.2.187 dropped `pread`/`pwrite` for vxworks, but std still imports them

[physwkim]

x86_64-wrs-vxworks can't build std with libc 0.2.187 or later. No SDK needed
to reproduce:

cargo new vxcheck && cd vxcheck
cargo +nightly check -Zbuild-std=std,panic_abort --target x86_64-wrs-vxworks

pread/pwrite were dropped from the vxworks module in #5129, but std still
imports them for this target (sys/fd/unix.rs:32,406), so two E0432. They're
still listed in libc-test/semver/vxworks.txt.

VxWorks defines neither: nm over the wrsdk-vxworks7-qemu-1.17.0 sysroot and
kernel finds nothing for them, so they'd have to be shims again, as they were up
to 0.2.186. A shim returns -1 without setting errno though, and std's cvt
reads last_os_error(), so maybe std shouldn't call them here at all.

Would you be open to bringing the shims back, or is this better fixed on the std
side?

(The same build also stops on killpg, but that one is only a std problem —
VxWorks has no process groups at all, so there's nothing for libc to declare:
rust-lang/rust#159969)

[JohnTitor]

These are just stub so I'd keep them removed. Like killpg, fixing std by returning unsupported would be better.

[tgross35]

Thanks for reporting; looks like this is covered in rust-lang/rust#159991 and rust-lang/rust#160031, which is indeed the right avenue.