Fix heap allocation when used with RTOS

Remove the code which checks the heap against the stack to determine
if there is space left.  Using the stack pointer as a limit causes
problems when used with an RTOS since the stack pointer depends
on the current thread which can use a user-allocated stack residing
anywhere in memory.

hal/common/retarget.cpp

@@ -461,9 +461,7 @@ extern "C" void __iar_argc_argv() {
 // Linker defined symbol used by _sbrk to indicate where heap should start.
 extern "C" int __end__;
 
-#if defined(TARGET_CORTEX_A)
 extern "C" uint32_t  __HeapLimit;
-#endif
 
 // Turn off the errno macro and use actual global variable instead.
 #undef errno
@@ -486,11 +484,7 @@ extern "C" caddr_t _sbrk(int incr) {
     unsigned char*        prev_heap = heap;
     unsigned char*        new_heap = heap + incr;
 
-#if defined(TARGET_CORTEX_A)
     if (new_heap >= (unsigned char*)&__HeapLimit) {     /* __HeapLimit is end of heap section */
-#else
-    if (new_heap >= (unsigned char*)__current_sp()) {
-#endif
         errno = ENOMEM;
         return (caddr_t)-1;
     }

hal/targets/cmsis/TARGET_Freescale/TARGET_MCU_K64F/TOOLCHAIN_GCC_ARM/startup_MK64F12.S

@@ -64,7 +64,7 @@ __StackTop:
 #ifdef __HEAP_SIZE
     .equ    Heap_Size, __HEAP_SIZE
 #else
-    .equ    Heap_Size, 0x400
+    .equ    Heap_Size, 0x20000
 #endif
     .globl    __HeapBase
     .globl    __HeapLimit