Fix up builds on Linux hosts after llvm 21.1.8 merge

This is because contrib/llvm-project/llvm/lib/Support/Unix/Threading.inc
attempts to use pthread_get_name_np(3) and pthread_set_name_np(3), which
are not defined on Linux.

Reported by: ivy
PR: 292067
MFC after: 1 month

(cherry picked from commit b6d823e39072daae9df4b47a15ea7142b7a55f1a)