main: Explicitly set the stack top based on _estack

.. setting it based on the ad-hoc stack pointer calculation of
mp_stack_ctrl_init() meant that the stack used above main() counts
against the 1KiB safety factor that the mp_stack_set_limit call tries
to establish.  It turns out, at least on M4, that over half of the
safety factor is used up by stack-above-main()!

In the case of the basics/gen_stack_overflow.py test,
which blows the stack on purpose, it turns out that gc would be called
while handling the "maximum recursion depth exceeded" error, and this
needed more stack than was left.

Closes: #900

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