Some tier 2 platforms not available anymore on rustup

[pietroalbini]

These tier 2 platforms have no builds on stable, and are marked as available = false in the manifest. Since these are tier 2 we should ship binaries for them (without guaranteeing they work).

• aarch64-unknown-cloudabi
• armv7-unknown-cloudabi-eabihf
• i686-unknown-cloudabi
• powerpc-unknown-linux-gnuspe (not in the platform support, but I guess tier 2)
• sparc-unknown-linux-gnu (not in the platform support, but I guess tier 2)

[aidanhs]

We've decided that these have probably been missing forever and we're fine demoting to Tier '2.5' on the forge.

[aidanhs]

Discussion: https://mozilla.logbot.info/rust-infra/20180731#c15097689-c15097740

[aidanhs]

Demoted in https://forge.rust-lang.org/platform-support.html

[RalfJung]

Given that the forge says "This status is accidental", shouldn't there an issue somewhere that tracks getting the remaining "tier 2.5" platforms out of that tier?