`thread_measure_stack_free()` previously assumed that reading past the
stack is safe. When the stack was indeed part of a thread, the
`thread_t` structure is put after the stack, increasing the odds of
this assumption to hold. However, `thread_measure_stack_free()` could
also be used on the ISR stack, which may be allocated at the end of
SRAM.
A second parameter had to be added to indicate the stack size, so that
reading past the stack can now be prevented.
This also makes valgrind happy on `native`/`native64`.
Since fmt no longer has a significant advantage in stack consumption,
we need to bump the `MIN_SIZE` guard that prevents causing stack
overflows due to the printing of the stack consumption.
