Move MacOS CI tests to GitHub Actions; add M1 CI tests

[SWilson4]

This PR moves our macOS CI tests from CircleCI to GitHub Actions. It also takes advantage of the new M1 runner. I believe this promotes macOS on aarch64 to a "Tier 1" platform.

Fixes #1630.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider and OQS-OpenSSH will also need to be ready for review and merge by the time this is merged.)

[Martyrshot]

Nice PR! Just a quick question about macos x86. :)

[baentsch]

Nice PR! Just a quick question about macos x86. :)

Well, what about (Apple on) PowerPC? :-)

[baentsch]

Thanks very much! LGTM

[Martyrshot]

Nice PR! Just a quick question about macos x86. :)

Well, what about (Apple on) PowerPC? :-)

X86_64 is still supported by all MacOS releases. PowerPC is not. :)