Rollup merge of #138409 - xingxue-ibm:use-sigaction, r=compiler-errors

Use sa_sigaction instead of sa_union.__su_sigaction for AIX

Revert test cases to use `sa_sigaction` instead of `sa_union.__su_sigaction`, now that the `libc` crate implementation for AIX defines `sa_sigaction` as a direct member of `struct sigaction`, aligning it with implementations on other similar platforms. ([[AIX] Use sa_sigaction instead of the union](rust-lang/libc#4250)).

Author: Manishearth