Use ln(1) rather than cp(1) to set up host /system for testing.

The trouble with cp is that if you're running a command directly (because
you're using gdb, say), you need to make sure you've copied the linker
and shell again if you've done a rebuild that caused a change there.

Change-Id: I312da30c0f92b515967e854e0faba272c961ecc7