grpc: py-grpcio port build failures for arm

[mascguy (Christopher Nielsen)]

Some rather, well, interesting build errors, are occurring on our ARM buildbot:

[...]
clang: error: no such file or directory: '"C"'
clang: error: no such file or directory: '__attribute__((visibility'
clang: error: no such file or directory: '("default")))'
clang: error: no such file or directory: 'PyObject*'
clang: error: clangno such file or directory: '("default")))': error: no such file or directory: '("default")))'
[...etc...]

That's from this example, for py39-grpcio: ​https://build.macports.org/builders/ports-11_arm64-builder/builds/20741/steps/install-port/logs/stdio

Full log also attached; filename: py39-grpcio-buildbot-failure-arm.txt.gz

Is it possible that we have a bug in one of our portgroups?