Rollup merge of rust-lang#147870 - devnexen:vxworks_simpl_cpu_detect, r=joboet

`std::thread::available_parallelism()` vxworks libc symbol usage.

Author: Zalathar

std/src/sys/thread/unix.rs

@@ -314,13 +314,10 @@ pub fn available_parallelism() -> io::Result<NonZero<usize>> {
         target_os = "vxworks" => {
             // Note: there is also `vxCpuConfiguredGet`, closer to _SC_NPROCESSORS_CONF
             // expectations than the actual cores availability.
-            unsafe extern "C" {
-                fn vxCpuEnabledGet() -> libc::cpuset_t;
-            }
 
             // SAFETY: `vxCpuEnabledGet` always fetches a mask with at least one bit set
             unsafe{
-                let set = vxCpuEnabledGet();
+                let set = libc::vxCpuEnabledGet();
                 Ok(NonZero::new_unchecked(set.count_ones() as usize))
             }
         }