Corrected Building.md and added additional content.

[moluopro]

The following error or outdated content have been fixed:

1. none is not an allowed value for option --compiler.
2. --report and --checked are no longer in use.

moluo@ubuntu:~/work/dart-sdk/sdk$ ./tools/test.py \
  --compiler none \
  --runtime vm \
  --progress color \
  --arch x64 \
  --mode release \
  --checked \
  --report \
  --time \
  --tasks 6 \
  language
"none" is not an allowed value for option "--compiler".
moluo@ubuntu:~/work/dart-sdk/sdk$ ./tools/test.py --help | grep report
moluo@ubuntu:~/work/dart-sdk/sdk$ ./tools/test.py --help | grep check
moluo@ubuntu:~/work/dart-sdk/sdk$ 

Added some explanations for the parameters of test.py in Building.md. Additionally, a small section on cross-compilation test-items has been added to Building-Dart-SDK-for-ARM-or-RISC-V.md.

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[copybara-service]

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[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/410446 has been updated with the latest commits from this pull request.