BUILD: Update For IAR support

Applied the same change as in mbed-crypto for using this as a sub
project with the IAR toolchain. Use __asm generic ,and avoid empty
enum. Avoid declaration of array with null size. This is a porting
of the original patch contributed to trusted-firmware-m.

Signed-off-by: TTornblom <thomas.tornblom@iar.com>
Signed-off-by: Michel Jaouen <michel.jaouen@st.com>
Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 9092c49..4d7e0b0 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -226,7 +226,9 @@
 endif(CMAKE_COMPILER_IS_CLANG)
 
 if(CMAKE_COMPILER_IS_IAR)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warn_about_c_style_casts -Ohz")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warn_about_c_style_casts")
+    set(CMAKE_C_FLAGS_RELEASE     "-Ohz")
+    set(CMAKE_C_FLAGS_DEBUG       "--debug -On")
 endif(CMAKE_COMPILER_IS_IAR)
 
 if(CMAKE_COMPILER_IS_MSVC)