It's incompatible with CRuby, but it is required for mruby because
it cannot detect `break` outside of loops in the parsing time, but
in the code generation time where it cannot raise `SyntaxError`.

#3711 fix

Save gc_arena after buffer got allocated

Latest fix was freeing the buffer while it was still in use.
Fixes #3712

This patch fixes the latter part of #3710. We need to change
`struct REnv` to fix the former part of the issue.

The behavior of `OP_RAISE` has been changed.

Fix a typo in gc.c.

Arena can be modified during the marking phase.

Because the root context cannot be marked from fibers.

Mostly for debugging purpose.

Otherwise the function may terminate and cause memory leaks.

This means #3701 is now OK to merge.