libc: fix reg32 on 32 bit freebsd

[mbyx]

Description

Attempts to fix errors in freebsd i686 by making sure that the correct build target is used when expanding macros.

Sources

https://cgit.freebsd.org/src/tree/sys/x86/include/reg.h?h=releng/13.1

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

[mbyx]

There's only one reg32 struct in libc, and it should only be included if the target is 64 bit, but it fails on i686 which is a 32 bit platform. Looking at the .cirrus.yml file, I found the evidence: # Test i686 FreeBSD in 32-bit emulation on a 64-bit host.

What's happening is that libc is being expanded as if it was 64 bit, causing this problem.

I'm not sure but we might be able to fix this if we pass in the correct target when expanding macros?

[mbyx]

It worked! (FreeBSD 13 still likes to segfault, we might have to rerun multiple times, but it does work now)