Change the "thread" test main for Cortex-A

In pararell threads of "thread" test is a simple thread, but it seems that there is a difference in the stack used between Cortex-A and Cortex-M.
As a result of check, in Cortex-A, program aborts because of the lack of stack when PARALLEL_THREAD_STACK_SIZE is 384, workes properly when PARALLEL_THREAD_STACK_SIZE is 512 at least.

TESTS/mbedmicro-rtos-mbed/threads/main.cpp

@@ -26,7 +26,11 @@
 #endif
 
 #define THREAD_STACK_SIZE 512
+#if defined(__CORTEX_A9)
+#define PARALLEL_THREAD_STACK_SIZE 512
+#else
 #define PARALLEL_THREAD_STACK_SIZE 384
+#endif
 #define CHILD_THREAD_STACK_SIZE 384
 
 using namespace utest::v1;