CMake: Fix Mbed TLS compilation for Cortex-M0/0+/1/M23

Due to a known issue in Mbed TLS's architecture determination
(ARMmbed/mbedtls#1077), we get the error

    error: inline assembly requires more registers than available

when compiling `bignum.c` for Cortex-M0/0+/1/M23 which do not have
the macro `__thumb2__` set by the compiler.

The workaround is to define the macro `MULADDC_CANNOT_USE_R7` which
is already defined by Mbed CLI 1 but missing in our CMake support.

Fixes ARMmbed/mbed-os-example-lorawan#220

connectivity/mbedtls/CMakeLists.txt

@@ -102,3 +102,20 @@ target_compile_definitions(mbed-mbedtls
     INTERFACE
         MBED_CONF_MBEDTLS_PRESENT=1
 )
+
+# Workaround for https://github.com/ARMmbed/mbedtls/issues/1077
+# which affects cores without __thumb2__ set by the compiler
+# due to the lack of full Thumb-2 support
+set(partial-thumb2-cores
+    Cortex-M0
+    Cortex-M0+
+    Cortex-M1
+    Cortex-M23
+    Cortex-M23-NS
+)
+if(MBED_CPU_CORE IN_LIST partial-thumb2-cores)
+    target_compile_definitions(mbed-mbedtls
+        INTERFACE
+            MULADDC_CANNOT_USE_R7
+    )
+endif()

tools/cmake/profiles/debug.cmake

@@ -78,7 +78,6 @@ function(mbed_set_profile_options target mbed_toolchain)
         target_compile_definitions(${target}
             INTERFACE
                 __ASSERT_MSG
-                MULADDC_CANNOT_USE_R7
         )
     endif()