Libsystem: Replace cpio with copyHierarchy

I noticed every time building Libsystem that there was a very slow phase
where the output consisted of many lines like `42 blocks`. This is
output from cpio and in brief testing it takes at least twice as long to
go through cpio for the simple copies we want to make. The interface is
very convenient but the performance penalty is very painful so I decided
to implement a function that emulates cpio's interface.

On my machine with a 7200 RPM HDD this speeds up the build from about
250 minutes to about 75 seconds, a factor of 200! I'm not certain the
results on modern hardware would be similarly positive so this could use
further testing.

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