Intended change in behavior: Remove support for ARMv6 in Dart VM.

Justification/rationale: Flutter doesn't officially support ARMv6 (see also flutter faq) and neither does Dart. We have no test coverage of it either (afaik).

Expected impact of this change: Should not impact any direct users of Flutter / Dart SDK. It will no longer be possible build custom Dart VM for ARMv6 and run on ARMv6 devices (known affected devices: old Raspberry Pi 1 & Zero).

Clear steps for mitigating the change: None

/cc @Hixie @vsmenon @mit-mit @a-siva @franklinyow