Mark mutex as robust to prevent deadlocks (#1233)

Mark mutex as robust to prevent deadlocks (#1233)

Mark mutex as robust to prevent deadlocks

Prevent application hangs that occur when a thread dies while holding a
mutex, particularly during vTaskEndScheduler or exit calls. This is
achieved by setting the PTHREAD_MUTEX_ROBUST attribute on the mutex.

Fixes:
- GitHub issue: FreeRTOS/FreeRTOS-Kernel#1217
- Forum thread: freertos.org/t/22287

Signed-off-by: Gaurav Aggarwal <aggarg@amazon.com>