unix: add Prctl for linux platforms

Adds Prctl syscall for linux platforms. syscall and x/sys/unix already
defined the PR_ series of constants.

See discussion: https://groups.google.com/forum/#!topic/golang-nuts/rG6xShds2Dc

Change-Id: Ida5562b46abef574a3f9ac31911142f1e8d7fb4f
Reviewed-on: https://go-review.googlesource.com/15520
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux.go

@@ -886,6 +886,7 @@ func Getpgrp() (pid int) {
 //sys	Pause() (err error)
 //sys	PivotRoot(newroot string, putold string) (err error) = SYS_PIVOT_ROOT
 //sysnb prlimit(pid int, resource int, old *Rlimit, newlimit *Rlimit) (err error) = SYS_PRLIMIT64
+//sys   Prctl(option int, arg2 uintptr, arg3 uintptr, arg4 uintptr, arg5 uintptr) (err error)
 //sys	read(fd int, p []byte) (n int, err error)
 //sys	Removexattr(path string, attr string) (err error)
 //sys	Renameat(olddirfd int, oldpath string, newdirfd int, newpath string) (err error)
@@ -1022,7 +1023,6 @@ func Munmap(b []byte) (err error) {
 // Personality
 // Poll
 // Ppoll
-// Prctl
 // Pselect6
 // Ptrace
 // Putpmsg