rdmd_test.d: Make model flag optional

[the-horo]

The -m switch only makes sense on amd64 or x86. Making the flag optional allows running the tests on other architectures.

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[CyberShadow]

Do we need to bump dlang-community/setup-dlang?

[the-horo]

I've made the PR in order to be able to run tests on aarch64. There, ldc2 handles -m64 and probably ignores it, but gdc fails with an unrecognized option. I've tried to keep the old behavior the same so I'm not sure why a bump would be needed.

[CyberShadow]

Makes sense, though my comment was directed at the failing jobs on macOS.

but gdc fails with an unrecognized option

My vague understanding is that the relationship between AArch64 and 32-bit ARM is roughly the same as between x86_64 and x86. Would it make sense to look into whether it would be better for GDC on ARM to have a -m flag as well?