.heap support

This adds a fictitious `.heap` linker section that denotes the span of a heap region. The size of
this linker section is governed by the `_heap_size` symbol which defaults to 0 but can be overridden
by the user.

Author: japaric

cortex-m-rt/link.x

@@ -52,7 +52,7 @@ SECTIONS
   _sstack = ORIGIN(RAM);
   _estack = _stack_start;
 
-  /* ficticious region that represents the memory available for the stack */
+  /* fictitious region that represents the memory available for the stack */
   .stack _sstack (INFO) : ALIGN(4)
   {
     . += (_estack - _sstack);
@@ -75,6 +75,17 @@ SECTIONS
     _edata = .;
   } > RAM AT > FLASH
 
+  PROVIDE(_heap_size = 0);
+
+  _sheap = _edata;
+  _eheap = _sheap + _heap_size;
+
+  /* fictitious region that represents the memory available for the heap */
+  .heap _sheap (INFO) : ALIGN(4)
+  {
+    . += _heap_size;
+  }
+
   /* fake output .got section */
   /* Dynamic relocations are unsupported. This section is only used to detect
      relocatable code in the input files and raise an error if relocatable code