cmake: Exclude utils from non-cross-compile builds

Exclude the utils folder when not cross-compiling. The functionality
that utils provides, like retargeting support, isn't needed for unit
testing. We likely want to unit test the code in the utils folder at
some point, but we don't at the moment, so we can exclude it for now.

Signed-off-by: Jaeden Amero <jaeden.amero@arm.com>