It's already Tier 2. [1]
*-unknown-linux-ohos:
Tier: 2 (with Host Tools): aarch64-unknown-linux-ohos, armv7-unknown-linux-ohos, x86_64-unknown-linux-ohos
Tier: 3: loongarch64-unknown-linux-ohos
OpenHarmony has no support for gcc. All the toolchains are LLVM. [2]
[1] https://doc.rust-lang.org/rustc/platform-support/openharmony...