M2351: Support GCC

1.  Enable GCC support on non-secure targets
2.  Disable GCC support on secure targets becasue of GCC bug (as of 9-2019-q4-major): In non-secure entry function, callee-saved registers must be restored, but they are incorrectly cleared at optimization level "Os".

Author: ccli8

targets/targets.json

@@ -12672,6 +12672,7 @@
         "trustzone": true,
         "supported_toolchains": [
             "ARMC6",
+            "GCC_ARM",
             "IAR"
         ],
         "extra_labels_add": [
@@ -12733,6 +12734,7 @@
         "core": "Cortex-M23-NS",
         "supported_toolchains": [
             "ARMC6",
+            "GCC_ARM",
             "IAR"
         ],
         "tfm.level": 1,