Add C++ support to Makefile, and proof-of-concept simple.cpp example

This is a work in progress based on @mikejac's work.

Missing:
* No 'new' operator.
* I don't think STL is currently supported.

core/sdk_compat.c

@@ -14,3 +14,18 @@ void IRAM *zalloc(size_t nbytes)
 {
     return calloc(1, nbytes);
 }
+
+extern void (*__init_array_start)(void);
+extern void (*__init_array_end)(void);
+
+/* Do things which should be done as part of the startup code, but aren't.
+
+   Can be replaced with _start() once we have open source startup code.
+*/
+void sdk_compat_initialise()
+{
+    /* Call C++ constructors or C functions marked with __attribute__((constructor)) */
+    void (**p)(void);
+    for ( p = &__init_array_start; p != &__init_array_end; ++p)
+        (*p)();
+}