use `qemu-sparc64` to run sparc64 tests

[folkertdev]

fixes https://github.com/rust-lang/libc/issues/4061

Description

uses qemu-sparc64 to run the sparc64 tests. That is (for some reason) much faster than the test-runner-linux script that uses qemu-system-sparc64 under the hood. It runs the test suite locally in ~3 minutes, similar to the other targets.

I added some things to make this work, I'm not sure if other things should be removed that are now no longer needed.

Sources

Checklist

• [x] Relevant tests in libc-test/semver have been updated
• [x] No placeholder or unstable values like *LAST or *MAX are included (see #3131)
• [x] Tested locally (cd libc-test && cargo test --target mytarget); especially relevant for platforms that may not be checked in CI

r? @tgross35

[folkertdev]

on CI it's 4:45 ish, still pretty good, and no longer the bottleneck for CI.

[tgross35]

Merged in 15 minutes, awesome improvement