tools/nolibc: Implement gettid()

Allow test programs to determine their thread ID.

Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>