Fix the tests on riscv64gc-unknown-freebsd

[asomers]

The fpregs was defined with two fields having the wrong sign, and one field having the wrong name.

Description

Fixes one struct that was defined incorrectly for FreeBSD riscv64

Sources

https://github.com/freebsd/freebsd-src/blob/7a3af393d8ac2dfe72d24fe401344b60c4f87866/sys/riscv/include/ucontext.h#L50

Checklist

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

[rustbot]

r? @tgross35

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[tgross35]

Do you want this in stable? This seems low risk enough that we could do so.

[asomers]

Do you want this in stable? This seems low risk enough that we could do so.

Sure. I'll test it locally first though. This will take about an hour 😢 .

[asomers]

@tgross35 tests pass. Go ahead and merge to libc-0.2.

[tgross35]

Thanks, I'll get it in the next round.