Determine/document support for "Intel oneAPI" compilers

[Paul Hargrove]

In addition to what Intel now calls their "classic C/C++" compiler, there is now a distinct C/C++ compiler as part of the oneAPI suite (free, fwiw). It is also known as the "DPC++" compiler.

These can be distinguished from the "classics" by the executables (icx and icpx) and by differences in their --version output. The former ensures that (other than via something like environment modules setting CC and CXX) a user will not build UPC++ with these compilers "by accident". The latter ensures that even if one does try CXX=icpx, we will warn at configure time that this compiler is unknown.

It would be nice to have some mention in the documentation in the 2021.3.0 release.
Even just "While initial testing has revealed no issues, it is still too early to claim support for this compiler family"

[Paul Hargrove]

@Colin MacLean has assured me that these compliers are open-source forks of Clang. As such, I plan to document them as not officially supported in the same vein we currently disclaim Arm Ltd.'s fork of clang. I plan to include AMD's fork in the same statement.

[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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