syscall: add PtraceSyscall(pid int, signal int)
syscall: add PtraceSyscall(pid int, signal int)

Fixes issue 3525.

PTRACE_SYSCALL behaves like PTRACE_CONT and can deliver
a signal to the process. Ideally PtraceSingleStep should
support the signal argument, but its interface is frozen
by Go1.

R=golang-dev, r, rsc
CC=golang-dev
http://codereview.appspot.com/6353051

tv42/old-go,dsriram/golang,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,gnanderson/go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go