std: make positioned I/O unsupported on VxWorks

[JohnTitor]

Currently, build-std for VxWorks fails because std calls pread/pwrite for positioned I/O.
Previously libc defined them as shims, but recently these were removed to align the actual state of SDK: rust-lang/libc#5129
It should make sense to return an unsupported error for VxWorks.
Fix rust-lang/libc#5328

[rustbot]

r? @Darksonn

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[Darksonn]

Hmm, this is a bit messy. The current code has isolated all the conditional logic to this cfg_select!, but this PR introduces conditional logic in a bunch of other places.

If you do this, you can keep the conditional logic in one place:

Suggested change

	cfg_select! {
	target_os = "vxworks" => {
	pub unsafe fn pread64(
	_fd: c_int,
	_buf: *mut c_void,
	_count: size_t,
	_offset: off64_t
	) -> ssize_t {
	-1
	}
	}
	any(
	all(target_os = "linux", not(target_env = "musl")),
	target_os = "android",
	target_os = "hurd",
	) => {
	// Prefer explicit pread64 for 64-bit offset independently of libc
	// #[cfg(gnu_file_offset_bits64)].
	use libc::pread64;
	}

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