Work around ARM C 6 constexpr assert

ARMC6's assert macro does not work in C++14 constexpr context as it should.
By defining __DO_NOT_LINK_PROMISE_WITH_ASSERT, we deactivate the
extension that breaks it (having `__promise` inside `assert` - see the
compiler manual).

The extension does not appear to be useful - we have no code using
ARMC6's `__promise` directly, and putting a `__promise` inside the
assert does not appear to affect code generation in my tests.

platform/mbed_toolchain.h

@@ -24,6 +24,15 @@
 #define __error_t_defined 1
 #endif
 
+/* Work around ARM Compiler 6 bug - assert does not work in constexpr
+ * functions unless you stop it from using its __promise built-in.
+ */
+#ifdef __ARMCC_VERSION
+#ifndef __DO_NOT_LINK_PROMISE_WITH_ASSERT
+#define __DO_NOT_LINK_PROMISE_WITH_ASSERT
+#endif
+#endif
+
 // Warning for unsupported compilers
 #if !defined(__GNUC__)   /* GCC        */ \
  && !defined(__clang__)  /* LLVM/Clang */ \