libc: thread local errno support

Store errno in pthread objects, return member upon call to '__error()'.
This became necessary in order to make errno thread-safe.

Note, any call to libc code from a non-pthread (beside the first
entrypoint) is not supported.

issue #3568

repos/libports/src/lib/libc/errno.cc

@@ -13,9 +13,9 @@
  * under the terms of the GNU Affero General Public License version 3.
  */
 
-static int private_errno;
+#include <internal/pthread.h>
 
 extern "C" int *__error(void)
 {
-	return &private_errno;
+	return &pthread_self()->thread_local_errno;
 }

repos/libports/src/lib/libc/internal/pthread.h

@@ -180,6 +180,8 @@ struct Libc::Pthread : Noncopyable, Thread::Tls::Base
 
 	public:
 
+		int thread_local_errno = 0;
+
 		/**
 		 * Constructor for threads created via 'pthread_create'
 		 */