Support finding executable dir on Darwin

QEMU searches for its datafiles in directories that are relative
to the executable directory. However, the function to find the
executable directory is currently not implemented on Darwin which
causes QEMU to fallback and look for the datafiles using absolute
paths which fails when using a prebuilt distribution.

Change-Id: Ibc67bf1fefbbca0fb62d4f209bffc27d32d59cca
1 file changed