rtos: Thread: Make stack allocation failure runtime catchable

When a Thread object's stack memory is not provided, its `start()`
member function dynamically allocates its stack from the heap. If
allocation fails, there is no way to catch it because
* `std::nothrow` is missing after the `new` keyword. As Mbed OS
is built with `-fno-exceptions` (C++ exceptions disabled), failed
allocation results in an unrecoverable fault.
* The attempted `nullptr` check, which doesn't work anyway due to
the first point, is an assertion instead of error returning.
Assertions should be used as a development tool to ensure code
behaves correctly. But out-of-memory is a completely valid runtime
situation.

This commit adds the missing `std::nothrow`, and makes `Thread::start()`
return `osErrorNoMemory` if allocation fails so the caller can handle
it.

Note: A case when a thread should never fail due to lack of memory
is the main thread. But the main thread's stack is a pre-allocated
array in the static memory, passed to the `Thread()` constructor
during thread creation, so it's not impacted by this change.

rtos/include/rtos/Thread.h

@@ -131,7 +131,8 @@ public:
 
     /** Starts a thread executing the specified function.
       @param   task           function to be executed by this thread.
-      @return  status code that indicates the execution status of the function.
+      @return  status code that indicates the execution status of the function,
+               or osErrorNoMemory if stack allocation failed.
       @note a thread can only be started once
 
       @note You cannot call this function ISR context.

rtos/source/Thread.cpp

@@ -81,8 +81,11 @@ osStatus Thread::start(mbed::Callback<void()> task)
     }
 
     if (_attr.stack_mem == nullptr) {
-        _attr.stack_mem = new uint32_t[_attr.stack_size / sizeof(uint32_t)];
-        MBED_ASSERT(_attr.stack_mem != nullptr);
+        _attr.stack_mem = new (std::nothrow) uint32_t[_attr.stack_size / sizeof(uint32_t)];
+        if (_attr.stack_mem == nullptr) {
+            _mutex.unlock();
+            return osErrorNoMemory;
+        }
     }
 
     //Fill the stack with a magic word for maximum usage checking