Do not chmod the symlink target when a prebuilt binary is symlinked.

It may be in a source tree that isn't writable. Instead check that
the target is executable.

Test: m
Test: chmod a-x system/sepolicy/tools/insertkeys.py && \
  rm -rf out/host out/soong/host && \
  m insertkeys.py
  Check that it fails with the expected error message.
Fixes: 169375644
Change-Id: I402d029dba45783127586ad9749c90012afe891d