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
diff --git a/app/CMakeLists.txt b/app/CMakeLists.txt
index abd6865..936473f 100755
--- a/app/CMakeLists.txt
+++ b/app/CMakeLists.txt
@@ -1,5 +1,6 @@
 #-------------------------------------------------------------------------------
 # Copyright (c) 2020-2021, Arm Limited. All rights reserved.
+# Copyright (c) 2022, Cypress Semiconductor Corp. All rights reserved.
 #
 # SPDX-License-Identifier: BSD-3-Clause
 #
@@ -187,11 +188,11 @@
     endif()
 
     if (NOT CPU_ARCH)
-        if (${CMAKE_SYSTEM_ARCHITECTURE} STREQUAL armv8-m.main)
+        if (${TFM_SYSTEM_ARCHITECTURE} STREQUAL armv8-m.main)
             set(CPU_ARCH armv8m_ml)
-        elseif (${CMAKE_SYSTEM_ARCHITECTURE} STREQUAL armv8-m.base)
+        elseif (${TFM_SYSTEM_ARCHITECTURE} STREQUAL armv8-m.base)
             set(CPU_ARCH armv8m_bl)
-        elseif (${CMAKE_SYSTEM_ARCHITECTURE} STREQUAL armv7-m)
+        elseif (${TFM_SYSTEM_ARCHITECTURE} STREQUAL armv7-m)
             set(CPU_ARCH armv7m)
         endif()
     endif()