uucore: Switch to unix_sigpipe attibute and drop SIGPIPE macros

[oech3]

We have MSRV 1.88 now. No need to maintain many macros.

[xtqqczze]

Isn't unix_sigpipe attribute unstable?

[oech3]

Oh no

[xtqqczze]

It's been replaced with -Zon-broken-pipe=... compiler flag.