Work around ARM C 6 constexpr assert
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.

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