Mark riscv64gc-unknown-linux-musl as tier 2 target

[mati865]

According to

rust/src/doc/rustc/src/platform-support/riscv64gc-unknown-linux-musl.md

Line 3 in cdb4236

**Tier: 2**

it's tier 2 target.

In case you are wondering how I noticed it:
Dockerfiles at Wild linker repo started to fail building recently due to missing riscv64gc-unknown-linux-musl std. I had hoped the problem would go away by itself, but it did not (it never does...).
#148983 happened recently, so I checked https://rust-lang.github.io/rustup-components-history/riscv64gc-unknown-linux-musl.html and yeah, the date matches.
Given this condition:

rust/src/tools/build-manifest/build.rs

Line 35 in cdb4236

if !(tier == 1 || tier == 2) {

I'm certain PR will fix the problem.

[rustbot]

These commits modify compiler targets.
(See the Target Tier Policy.)

[rustbot]

r? @WaffleLapkin

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[wesleywiser]

@bors r+ rollup

[bors]

📌 Commit fd1852f has been approved by wesleywiser

It is now in the queue for this repository.