Use returns_twice attribute to preserve regs in nlrthumb nlr_push()

[dhalbert]

• Fixes HardFault with DEBUG build on Metro M4 due to caught AttributeError #7261.

This program crashed on SAMD51 when compiled with DEBUG=1:

def crash():
    try:
        None.wrong()
    except AttributeError:
        pass
    i = None

The stack variable fastn in mp_execute_bytecode() in py/vm.c was getting smashed. It was a Heisenbug because adding print statements caused it to work.

After narrowing down the point of smashing, I looked at the assembly code and saw that a call to nlr_push() in mp_execute_bytecode assumed that register r3 was preserved, when it was not. nlr_push() is a "naked" function that does not have the usual prologue and postlogue.

This all seemed more and more familiar as I progressed, and it turns out I fixed this problem already, about five years ago, in #506! However, the fix got undone during an upstream MicroPython merge in 2021. The problem manifested differently and we didn't see this problem again for a while.

My #506 fix involved adding "don't clobber these registers" indicators to nlr_push(). There is a long discussion in the related issue, #500. A MicroPython developer discovered the same problem in a different context, but it never really got fixed in MicroPython, because it occurs due to LTO, and they rarely use LTO. See micropython#3889 (which was closed and not merged).

The fix proposed in micropython#3889 is to declare nlr_push() with the attribute returns_twice. That is simpler than my #506 fix, so I tried that this time, and lo and behold, it works.

Note that this does not solve #7279 (-O2 not working on SAMx5x).

[jepler]

This makes sense, thanks