Build: Alter handling of --cpu flag for armlink

To be compliant with correct syntax, and better handle cases related to
the new armclang support in cmake 3.18.

Change-Id: I21535cf3658ab22cd5f7d2714bf1b12e25eed075
Signed-off-by: Raef Coles <raef.coles@arm.com>
1 file changed