Tighten GCC 2-region _sbrk

When moving to the second heap region due to overflowing the first
region, the `_sbrk` implementation assumed the allocation would fit in
the second region, and didn't check for that overflowing too.

Problem revealed in `stats_heap` test with GCC 8 on K64F - the allocation
attempt for 1GiB crashed, as `_sbrk` indicated 1GiB was available at the
start of the second region.
second region.

Presumably older versions of newlib fault that allocation attempt before
passing to `_sbrk`.

While there, adjust the code to not use a separate static `bool`, saving
RAM. We can track with just one pointer, as order of the two regions is
fixed, and already relied on by newlib.

Author: kjbracey

platform/source/mbed_retarget.cpp

@@ -1268,27 +1268,26 @@ extern "C" WEAK void __cxa_pure_virtual(void)
 extern uint32_t __mbed_sbrk_start;
 extern uint32_t __mbed_krbs_start;
 /* Additional RAM memory used for heap - please note this
- * address should be lower address then the previous default address
+ * address must be lower address then the previous default address
  */
 extern uint32_t __mbed_sbrk_start_0;
 extern uint32_t __mbed_krbs_start_0;
 
 extern "C" WEAK caddr_t _sbrk(int incr)
 {
     static uint32_t heap = (uint32_t) &__mbed_sbrk_start_0;
-    static bool once = true;
     uint32_t prev_heap = heap;
     uint32_t new_heap = heap + incr;
 
     /**
-     * If the new address is outside the first region, start allocating from the second region.
-     * Jump to second region is done just once, and `static bool once` is used to keep track of that.
+     * If we exceed the first region, start allocating from the second region.
      */
-    if (once && (new_heap > (uint32_t) &__mbed_krbs_start_0)) {
-        once = false;
+    if (prev_heap <= (uint32_t) &__mbed_krbs_start_0 && new_heap > (uint32_t) &__mbed_krbs_start_0) {
         prev_heap = (uint32_t) &__mbed_sbrk_start;
         new_heap = prev_heap + incr;
-    } else if (new_heap > (uint32_t) &__mbed_krbs_start) {
+    }
+
+    if (new_heap > (uint32_t) &__mbed_krbs_start) {
         /**
         * If the new address is outside the second region, return out-of-memory.
         */