There’s no need to call posix_spawnp_setbinpref_np when we are launching in...

There’s no need to call posix_spawnp_setbinpref_np when we are launching in the shell.  That only sets the architecture of the shell, we use “arch —arch” to actually pick the binary we actually want to launch’s architecture.

<rdar://problem/16103187>

llvm-svn: 201831