Darwin: Use the same `ar` as Soong

Soong uses the copy of `ar` in the OSX SDK instead of the wrapper in
/usr/bin/ar. /usr/bin/ar appears to be a thin wrapper that looks up the
current SDK and passes execution to it. Soong does this so that it can
actually set up a dependency on the tool.

Change-Id: Ia4e4fbe3287539933fa98a1354c3ccee91f4d552