Determine/document support for "Apple silicon Mac"

[Paul Hargrove]

Apple is now marketing macs with their own ARM-based "Apple silicon" processors.
These require macOS 11 (see issue 410) and Xcode 12 (see issue 409).

We should have a clearly stated support status in our documentation in time for the next public release.

[Dan Bonachea]

Mass roll-over of open issues to next release milestone

[Paul Hargrove]

We now have regular automated testing (all results are good) of an Apple M1-based Mac mini.

A few GASNet patches were applied for this platform, but nothing is required in UPC++ for correctness.

What may be desired is explicit recognition of this platform in utils/system_checks.sh to remove the current warning about the platform being unrecognized. Whether we make those changes depends on what status we want to document.

[Paul Hargrove]

Proposed documentation appears in pull request 331

[Paul Hargrove]

Update INSTALL.md list of (un)supported platforms

This commit adds a list of NON supported platforms to INSTALL.md.

A "soft" disclaimer of support for macOS/aarch64 is added to the new list, resolving issue 411.

A blanket disclaimer for Clang and GCC forks from several vendors is added. This includes Intel's icx and icpx, and so this commit resolves issue 458. The pre-existing disclaimer of support for Arm Ltd.'s forks of Clang and GCC has been absorbed into this item.

A full disclaimer of support for NVIDIA-branded compilers is added. Unlike the case with Arm Ltd.'s compilers, this commit preserves the duplicate texts under the PGI compilers bullets for Linux/x86_64 and Linux/ppc64le.

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