This fixes an issue where if the build folder is no longer present flang
cannot link anything on Windows because the path to compiler-rt in the
binary is hard-coded. Flang already links compiler-rt on Windows so it
isn't necessary for flang-rt to specify that it depends on compiler-rt
at all, other than for the unit tests, so instead we can move that logic
into the unit test compile lines.
(cherry picked from commit c20a95a7ddd8219f3e587e335a0a8e3f4613fc47)
949c6cc95a