Change the "malloc" test main for Cortex-A

"malloc" test is used a simple thread that executes only malloc and free, 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 THREAD_STACK_SIZE is 256, workes properly when THREAD_STACK_SIZE is 512 at least. Since it seems that the definition processing of "DEFAULT_STACK_SIZE" was gone from Mbed OS by updating CMSIS5/RTX5, I setted 512 directly for Cortex-A.

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

@@ -32,7 +32,7 @@ volatile bool thread_should_continue = true;
 #define THREAD_MALLOC_SIZE  100
 
 #if defined(__CORTEX_A9)
-#define THREAD_STACK_SIZE   DEFAULT_STACK_SIZE
+#define THREAD_STACK_SIZE   512
 #else
 #define THREAD_STACK_SIZE   256
 #endif