stm: Provide malloc()/realloc()/free(), as forwarding to gc_*() functions.

Note: none of these functions are used by STM port, so they're provided
to have parity with pre gc refactor status quo, and in case they will be
needed in the future by some extension writers.

stm/mpconfigport.h

@@ -21,6 +21,14 @@ typedef float machine_float_t;
 
 machine_float_t machine_sqrt(machine_float_t x);
 
+// There is no classical C heap in bare-metal ports, only Python
+// garbage-collected heap. For completeness, emulate C heap via
+// GC heap. Note that MicroPython core never uses malloc() and friends,
+// so these defines are mostly to help extension module writers.
+#define malloc gc_alloc
+#define free gc_free
+#define realloc gc_realloc
+
 // board specific definitions
 
 // choose 1 of these boards

stm/std.h

@@ -2,10 +2,6 @@ typedef unsigned int size_t;
 
 void __assert_func(void);
 
-void *malloc(size_t n);
-void free(void *ptr);
-void *realloc(void *ptr, size_t n);
-
 void *memcpy(void *dest, const void *src, size_t n);
 void *memmove(void *dest, const void *src, size_t n);
 void *memset(void *s, int c, size_t n);