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 04c7cdb6 and
37865920.  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.