Rollup merge of rust-lang#87354 - Wind-River:2021_master, r=kennytm

Update VxWork's UNIX support

1. VxWorks does not provide glibc
2. VxWorks does provide `sigemptyset` and `sigaddset`

Note: these changes are concurrent to [this PR](rust-lang/libc#2295) in libc.

Author: JohnTitor

library/std/src/sys/unix/os/tests.rs

@@ -1,12 +1,14 @@
 use super::*;
 
 #[test]
+#[cfg(not(target_os = "vxworks"))]
 fn test_glibc_version() {
     // This mostly just tests that the weak linkage doesn't panic wildly...
     glibc_version();
 }
 
 #[test]
+#[cfg(not(target_os = "vxworks"))]
 fn test_parse_glibc_version() {
     let cases = [
         ("0.0", Some((0, 0))),

library/std/src/sys/unix/process/process_common.rs

@@ -50,7 +50,7 @@ cfg_if::cfg_if! {
             raw[bit / 8] |= 1 << (bit % 8);
             return 0;
         }
-    } else if #[cfg(not(target_os = "vxworks"))] {
+    } else {
         pub use libc::{sigemptyset, sigaddset};
     }
 }