Build: Fix PSA Arch test builds

Commit 866571c4d785a46aee41415bf47e6dc9e6e3a6cd
("Build: Add support for using target arch only") renamed
CMAKE_SYSTEM_ARCHITECTURE to TFM_SYSTEM_ARCHITECTURE.
Unfortunately, CMAKE_SYSTEM_ARCHITECTURE is still used in the
tf-m-tests repo.

Signed-off-by: Chris Brand <chris.brand@cypress.com>
Change-Id: Ie385a17ddc6d71fbfca544e3f7f8bba8f0517660
1 file changed