[lib][libc] Make errno thread safe using TLS APIs

Before this change, errno was "completely un-threadsafe" as the comment
states.

This changes errno to be threadsafe by making errno a thread local
variable.

kernel/include/kernel/thread.h

@@ -57,6 +57,7 @@ enum thread_tls_list {
 #ifdef WITH_LIB_LKUSER
     TLS_ENTRY_LKUSER,
 #endif
+    TLS_ENTRY_ERRNO,
     MAX_TLS_ENTRY
 };