Be more careful about symlinking a file that actually exists on the host.

Avoids this error on later runs:

  ln: cannot create symbolic link from '/sys/kernel/debug' to 'out/target/product/walleye/root/d/debug': Permission denied

Bug: http://b/145148158
Test: touch * ; mm -j32 -k
Change-Id: I0c1c0c6e7666c0bc3373687d8c1c7bdf0847913f