[tools] Fix list of possible arguments for -a in --help.
[tools] Fix list of possible arguments for -a in --help.

Make sure the architectures listed by --help for the -a command
line option reflect all currently available architectures + 'all'.

Change-Id: I2759c7c7e846d38fce5afde1854bfd2db0dd8b70
Reviewed-on: https://dart-review.googlesource.com/c/88821
Reviewed-by: Zach Anderson <9f8b31e960a68dade71618f1ff6da6e23a1edfc9@google.com>
Commit-Queue: Stevie Strickland <cce218d042444252b7ac446d60910fca7732559c@google.com>

dart-lang/sdk,dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk