test/sigchld.go: use syscall.Kill instead of a combination

of syscall.Syscall and syscall.SYS_KILL.

In RTEMS, there is no syscall.Syscall support, but it does
support POSIX signals. So, if this testcase is changed to use
syscall.Kill, then it would run fine on RTEMS, when using gccgo.

R=rsc, iant
CC=golang-dev
https://golang.org/cl/1863046

test/sigchld.go

@@ -10,6 +10,6 @@ package main
 import "syscall"
 
 func main() {
-	syscall.Syscall(syscall.SYS_KILL, uintptr(syscall.Getpid()), syscall.SIGCHLD, 0);
+	syscall.Kill(syscall.Getpid(), syscall.SIGCHLD);
 	println("survived SIGCHLD");
 }