process.c: fix failed status with spawnv

* process.c (proc_spawn_cmd_internal, proc_spawn_sh): spawn() with
  P_NOWAIT returns process ID, so it must not be set as a status.

* process.c (rb_spawn_process): set failed status on error on
  platforms using spawnv not only on Windows.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40552 b2dd03c8-39d4-4d8f-98ff-823fe69b080e