Enable f16 tests on loongarch

[tgross35]

Loongarch previously had a selection failure for f16 math llvm/llvm-project#93894. This was fixed in llvm/llvm-project#94456, which Rust got with the update to LLVM 19 #127513.

Enable loongarch in std/build.rs so we start running tests.

try-job: dist-loongarch64-linux
try-job: dist-loongarch64-musl

[rustbot]

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[tgross35]

Might get a merge conflict with #128349, but we can start this rolling. Cc @heiher

@bors try

[bors]

⌛ Trying commit 30641f0 with merge 247fa99...

[tgross35]

Cc also @xen0n since you posted llvm/llvm-project#94456

[bors]

☀️ Try build successful - checks-actions
Build commit: 247fa99 (247fa9906abe0d2416ce31fe15ac2ac518f8b89e)

[beetrees]

The f16 tests won't pass for loongarch64-unknown-linux-gnu as the LoongArch backend is affected by llvm/llvm-project#97981 (and llvm/llvm-project#97975). (The tests aren't run in CI for LoongArch.)

[tgross35]

That's right, I was forgetting that there was more to the story than just the crash. I will update the comment in #129385, otherwise I guess there isn't much to be done here.