Build: fix ARMClang _compiler_reload in cmake 3.18

Unset some more internal variables added in cmake 3.18 to allow ARMClang
compiler reloading to work correctly.

Change-Id: Ieff1e8b17057fc74bf426035e7baa1228a2a2394
Signed-off-by: Raef Coles <raef.coles@arm.com>
1 file changed