syscall: fix StartProcess in Plan 9

This makes os_test.TestStartProcess test from os package pass.

R=paulzhol, r2, r
CC=golang-dev
https://golang.org/cl/4385052

src/pkg/syscall/exec_plan9.go

@@ -363,7 +363,7 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err Error)
 	p[1] = -1
 
 	// Convert args to C form.
-	argv0p := StringBytePtr(argv[0])
+	argv0p := StringBytePtr(argv0)
 	argvp := StringArrayPtr(argv)
 
 	var chroot *byte