appveyor.yml: engage VC-WIN64A-masm.

One of the reasons for why masm/ml64 is not [fully] supported is that
it's problematic to support multiple versions. But latest one usually
works and/or it's lesser problem to make it work. So idea here is to
have a "whistle" when it breaks, so that problems can be evaluated as
they emerge. It's kind of "best effort" thing, as opposite to "full
support".

Reviewed-by: Richard Levitte <levitte@openssl.org>

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