As of https://go.dev/cl/476695 the package golang.org/x/sys/unix
expects a syscall.prlimit function to exist.  This libgo patch adds
that function.  This is for https://go.dev/issue/46279 and
https://go.dev/issue/59712.  Since this is a small patch and is needed
to compile the widely used x/sys/unix package, committed to tip and to
GCC 11, 12, and 13 branches.

Ian