openbsd: fix thread name buffer size

OpenBSD 7.3 changed its implementation of
pthread_get_name_np/pthread_set_name_np to wrap new libc functions
getthrname/setthrname and lowered the max buffer size from 32 to 24.

This is not a backwards-compatible change because if we were to put in
comptime version logic to use size 32 when target < 7.3 the binaries
would be undefined when running on >= 7.3. It also could simply be that
OpenBSD has a policy to not support older binaries running on newer
releases? Regardless, the safest course is to simply use the smallest
known buffer size.

As an aside, this bug manifested as a "hung" std.Thread test because 7.3
pthread API never checks for error result when wrapping getthrname/setthrname.
This is not a problem in std.Thread when we use the correct max buffer
size because ERANGE/EINVAL become unreachable.

lib/std/Thread.zig

@@ -40,7 +40,7 @@ pub const max_name_len = switch (target.os.tag) {
     .macos, .ios, .watchos, .tvos => 63,
     .netbsd => 31,
     .freebsd => 15,
-    .openbsd => 31,
+    .openbsd => 23,
     .dragonfly => 1023,
     .solaris => 31,
     else => 0,