Apollo3: Fix heap size formula and stack start address in scatter file

The heapsize was incorrectly calculated.
This fixes it by subtracting the Stack size, any memory chunks allocated
before the start of the application (for vectors and/or crash report), and
finally the size of the application from the total RAM size.

The stack start address should be the top of the RAM which is also fixed.

Author: hugueskamba

targets/TARGET_Ambiq_Micro/TARGET_Apollo3/TOOLCHAIN_ARM_STD/AMA3B1KK.sct

@@ -56,8 +56,8 @@ LR_IROM1 MBED_APP_START MBED_APP_SIZE {
   RW_IRAM1 MBED_RAM1_START MBED_RAM1_SIZE {
    .ANY (+RW +ZI)
   }
-  ARM_LIB_HEAP  AlignExpr(+0, 16)  EMPTY  (MBED_RAM_SIZE-RAM_FIXED_SIZE+MBED_RAM_START-AlignExpr(ImageLimit(RW_IRAM1), 16))  {
+  ARM_LIB_HEAP  AlignExpr(+0, 16)  EMPTY  (MBED_RAM_SIZE-RAM_FIXED_SIZE+MBED_RAM1_START-AlignExpr(ImageLimit(RW_IRAM1), 16))  {
   }
-  ARM_LIB_STACK MBED_RAM1_START+MBED_RAM1_SIZE-8 EMPTY -Stack_Size { ; Stack region growing down
+  ARM_LIB_STACK MBED_RAM1_START+MBED_RAM1_SIZE EMPTY -Stack_Size { ; Stack region growing down
   }
 }