targets/gba: make linker script cleaner

Make it clearer where the stack is located. Additionally, get the heap
to work (the GC needs to have _stack_top defined to work correctly).

targets/gameboy-advance.ld

@@ -7,10 +7,8 @@ MEMORY {
     rom     : ORIGIN = 0x08000000, LENGTH = 32M  /* flash ROM */
 }
 
-__iwram_top = ORIGIN(iwram) + LENGTH(iwram);;
-_stack_size = 3K;
-__sp_irq    = _stack_top;
-__sp_usr    = _stack_top - 1K;
+__stack_size_irq = 1K;
+__stack_size_usr = 2K;
 
 SECTIONS
 {
@@ -35,8 +33,10 @@ SECTIONS
     .stack (NOLOAD) :
     {
         . = ALIGN(4);
-        . += _stack_size;
+        _stack_top_irq = .;
+        . += __stack_size_irq;
         _stack_top = .;
+        . += __stack_size_usr;
     } >iwram
 
     /* Start address (in flash) of .data, used by startup code. */

targets/gameboy-advance.s

@@ -22,10 +22,10 @@ start_vector:
 
     mov     r0, #0x12                      // Switch to IRQ Mode
     msr     cpsr, r0
-    ldr     sp, =__sp_irq                  // Set IRQ stack
+    ldr     sp, =_stack_top_irq            // Set IRQ stack
     mov     r0, #0x1f                      // Switch to System Mode
     msr     cpsr, r0
-    ldr     sp, =__sp_usr                  // Set user stack
+    ldr     sp, =_stack_top                // Set user stack
 
     // Jump to user code (switching to Thumb mode)
     ldr     r3, =main