Summary: implement the right semantics: 1. getpid should return the main thread id 2. gettid should return the current thread id Refer to: https://github.com/apache/incubator-nuttx/issues/2499 https://github.com/apache/incubator-nuttx/pull/2518 Signed-off-by: qinwei1 <qinwei1@xiaomi.com>