pthread_setname_np() is not provided by the Fuchsia C library
so do not try to call it in Thread::setName().

Change-Id: I7db37e6da52d665ff6535d735e9a2da5d9845143
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/35930
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: David Turner <digit@google.com>
Presubmit-Ready: David Turner <digit@google.com>