Fix errno checking for dalvik-cache creation.

On the host, it's possible for /tmp/foo/dalvik-cache/<isa> to
not exist, but for /tmp/foo/dalvik-cache to be already present.
In this scenario, the first mkdir() can fail with EEXIST.

Change-Id: Ia99f35a28f5ce4e3ac6c47933b79369240683091