stm32/gccollect: Always use MP_STATE_THREAD(stack_top) to get stack top.

In a non-thread build, using &_ram_end as the top-of-stack is no longer
correct because the stack is not always at the very top end of RAM.  See
eg 04c7cdb668 and
3786592097.  The correct value to use is
&_estack, which is the value stored in MP_STATE_THREAD(stack_top), and
using the same code for both thread and non-thread builds makes the code
cleaner.

ports/stm32/gccollect.c

@@ -48,11 +48,7 @@ void gc_collect(void) {
     uintptr_t sp = gc_helper_get_regs_and_sp(regs);
 
     // trace the stack, including the registers (since they live on the stack in this function)
-    #if MICROPY_PY_THREAD
     gc_collect_root((void**)sp, ((uint32_t)MP_STATE_THREAD(stack_top) - sp) / sizeof(uint32_t));
-    #else
-    gc_collect_root((void**)sp, ((uint32_t)&_ram_end - sp) / sizeof(uint32_t));
-    #endif
 
     // trace root pointers from any threads
     #if MICROPY_PY_THREAD