Add the locks and flags necessary to make the IAR standard library
thread safe. These changes consist of:
-Add compiler flag "--guard_calls" to ensure C++ function-static
variables with dynamic initializers are initialized in a
thread safe manner
-Add the linker flag "--threaded_lib" so the thread safe version of
the standard library is used
-Implement mutex functions required for IAR thread safety
-Create a set of stub functions in retarget.c for when the rtos is not present
Added an #ifndef directive to the __MBED_CMSIS_RTOS_CM and __CMSIS_RTOS macro definitions in order to prevent "Incompatible redefinition of macro" warnings from the online compiler.
- Allows threads to started separately from when they are declared,
avoiding the need to dynamically allocate threads at runtime.
- Allows multiple threads to join without forceful termination. Allowing
threads to synchronize on cleanup.