Bump libc to 0.2.171 to use sigaction for AIX

[xingxue-ibm]

Change to use sa_sigaction instead of sa_union.__su_sigaction, now that the libc crate 0.2.171 implementation for AIX defines sa_sigaction as a direct member of struct sigaction, aligning it with implementations on other similar platforms. (rust-lang/libc#4250).

[vorner]

Hello

The changes look nice. However, you update the libc only in Cargo.lock ‒ which is only internal (for development) and downstream crates don't take it into account at all. Can you also mandate the right minimal libc in Cargo.toml?

[xingxue-ibm]

Hello

The changes look nice. However, you update the libc only in Cargo.lock ‒ which is only internal (for development) and downstream crates don't take it into account at all. Can you also mandate the right minimal libc in Cargo.toml?

Thanks for pointing this out! Changed Cargo.toml to mandate minimal libc version 0.2.171.