cmake: Fully-qualified path to ar

We (correctly enough) pass in our AR to cmake via the toolchain file,
but unlike the other cross-compilation tools (CC, CXX, ranlib, etc),
cmake does not then do its own search in the PATH for our AR.

By using cmake's own find_program() to search the PATH,
make works as expected from the commandline, outside of bitbake.

Signed-off-by: Douglas Royds <douglas.royds@taitradio.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>