Move sys::vxworks code to sys::unix

[CDirkx]

Follow-up to #77666, sys::vxworks is almost identical to sys::unix, the only differences are the rand, thread_local_dtor, and process implementation. Since vxworks is target_family = unix anyway, there is no reason for the code not to live inside of sys::unix like all the other unix-OSes.

rust/compiler/rustc_target/src/spec/vxworks_base.rs

Line 12 in e41f378

os_family: Some("unix".to_string()),

@rustbot label: +T-libs-impl