syscall: add prlimit

As of https://go.dev/cl/476695 golang.org/x/sys/unix can call
syscall.prlimit, so we need such a function in libgo.

For golang/go#46279
Fixes golang/go#59712

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/486576

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-0411a2733fd468e69f1998edd91e8fe3ba40ff9e
+737de90a63002d4872b19772a7116404ee5815b4
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/go/syscall/libcall_linux.go

@@ -189,6 +189,14 @@ func Gettid() (tid int) {
 //sys	PivotRoot(newroot string, putold string) (err error)
 //pivot_root(newroot *byte, putold *byte) _C_int
 
+// Used by golang.org/x/sys/unix.
+//sys	prlimit(pid int, resource int, newlimit *Rlimit, oldlimit *Rlimit) (err error)
+//prlimit(pid Pid_t, resource _C_int, newlimit *Rlimit, oldlimit *Rlimit) _C_int
+
+func Prlimit(pid int, resource int, newlimit *Rlimit, oldlimit *Rlimit) error {
+	return prlimit(pid, resource, newlimit, oldlimit)
+}
+
 //sys	Removexattr(path string, attr string) (err error)
 //removexattr(path *byte, name *byte) _C_int