[AIX] Use sa_sigaction instead of the union

[xingxue-ibm]

Description

The current libc crate implementation for AIX defines struct sigaction as containing a union sa_union with two members: __su_handler and __su_sigaction. This mirrors the struct sigaction definition in the AIX system header. Consequently, any reference to sa_sigaction in Rust code, whether in shipped crates or user code, must be replaced with sa_union.__su_sigaction for AIX. Additionally, the types of these two union members are declared as function pointers rather than sighandler_t (i.e., usize), as is the case on other platforms. This discrepancy causes compiler errors when sighandler_t is used as the type for signal handlers. Other operating systems, such as Linux, also define a union for sa_handler and sa_sigaction in struct sigaction. However, the libc crate implementations on these platforms define sa_sigaction directly as a member of struct sigaction rather than as part of a union. This patch modifies the libc crate implementation for AIX to define sa_sigaction as a direct member of struct sigaction, aligning it with implementations for other similar platforms. We will also update affected crates and test cases to reflect this change.

Sources

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]

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[xingxue-ibm]

close GH-4249

[daltenty]

LGTM from the AIX perspective. This makes the implementation common with what is done on other targets

[tgross35]

Is this something that should be changed once we are able to use unions (1.0)? If so, could you label the comment FIXME(union)?