py/nlr: Use MP_UNREACHABLE at the end of arch-specific nlr_jump funcs.

Recent versions of gcc perform optimisations which can lead to the
following code from the MP_NLR_JUMP_HEAD macro being omitted:

    top->ret_val = val; \
    MP_NLR_RESTORE_PYSTACK(top); \
    *_top_ptr = top->prev; \

This is noticeable (at least) in the unix coverage on x86-64 built with gcc
9.1.0.  This is because the nlr_jump function is marked as no-return, so
gcc deduces that the above code has no effect.

Adding MP_UNREACHABLE tells the compiler that the asm code may branch
elsewhere, and so it cannot optimise away the code.