mbed_retarget: Always provide __cxa_guard calls

We can use these retargeting calls to implement `mstd::call_once` for
all toolchains. (GCC and ARMC6 may have their own working
`std::call_once`, but IAR doesn't, so we do want this for IAR).

Author: kjbracey

platform/mbed_retarget.cpp

@@ -1490,8 +1490,6 @@ extern "C" void __env_unlock(struct _reent *_r)
 
 #endif
 
-#if defined (__GNUC__) || defined (__ARMCC_VERSION)
-
 #define CXA_GUARD_INIT_DONE             (1 << 0)
 #define CXA_GUARD_INIT_IN_PROGRESS      (1 << 1)
 #define CXA_GUARD_MASK                  (CXA_GUARD_INIT_DONE | CXA_GUARD_INIT_IN_PROGRESS)
@@ -1531,8 +1529,6 @@ extern "C" void __cxa_guard_abort(int *guard_object_p)
     singleton_unlock();
 }
 
-#endif
-
 #if defined(MBED_MEM_TRACING_ENABLED) && (defined(__ARMCC_VERSION) || defined(__ICCARM__))
 
 // If the memory tracing is enabled, the wrappers in mbed_alloc_wrappers.cpp