objspace_dump: Use FILE* to avoid crashing in mark functions

We observed crashes from rb_io_bufwrite() thread switching (through
rb_thread_check_ints()) in the middle of rb_execution_context_mark(). By
the time rb_execution_context_mark() gets a timeslice again, it read
garbage from a frame that was already popped in another thread, crashing
the process in SEGV. Other mark functions probably have their own ways
of breaking, but clearly, the usual IO code do too much for this
perilous pseudo GC context.

Use `FILE*` like before 5001cc47169614ea07d87651c95c2ee185e374e0
("Optimize ObjectSpace.dump_all"). Also, add type checking for
the private _dump methods.

Co-authored-by: Peter Zhu <peter@peterzhu.ca>

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