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