Warn about or prevent VLA use in C++

ARM and GNU compilers currently are in a mode where they will accept VLAs
in C++ as an extension. IAR does not accept them in C++.

Avoid potential portability surprises by making GCC warn, and
deactivating the extension in ArmCC.

tools/toolchains/arm.py

@@ -38,7 +38,7 @@ class ARM(mbedToolchain):
             "--brief_diagnostics", "--restrict", "--multibyte_chars", "-I \""+ARM_INC+"\""],
         'asm': [],
         'c': ["--md", "--no_depend_system_headers", "--c99", "-D__ASSERT_MSG"],
-        'cxx': ["--cpp", "--no_rtti"],
+        'cxx': ["--cpp", "--no_rtti", "--no_vla"],
         'ld': [],
     }
 

tools/toolchains/gcc.py

@@ -38,7 +38,7 @@ class GCC(mbedToolchain):
             ],
         'asm': ["-x", "assembler-with-cpp"],
         'c': ["-std=gnu99"],
-        'cxx': ["-std=gnu++98", "-fno-rtti"],
+        'cxx': ["-std=gnu++98", "-fno-rtti", "-Wvla"],
         'ld': ["-Wl,--gc-sections", "-Wl,--wrap,main",
             "-Wl,--wrap,_malloc_r", "-Wl,--wrap,_free_r", "-Wl,--wrap,_realloc_r"],
     }