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Revert to trival pow() optimizations to prevent inaccuracies.

(cherry-picked from commit 96d6d5032)

This patch fixes different misbehaviours between JIT-compiled code and
the interpreter for power operator in the following ways:
* Drop folding optimizations for base ^ n => base * base ..., as far as
  pow(base, n) isn't interchangeable with just multiplicity of numbers
  and depends on the <math.h> implementation.
* Since the internal power function is inaccurate for very big or small
  powers, it is dropped, and `pow()` from the standard library is used
  instead. To save consistency between JIT behaviour and the VM,
  narrowing optimization is dropped, and only trivial folding
  optimizations are used. Also, `math_extern2` version with two
  parameters is dropped, since it's no longer used.

Also, this fixes failures of the [220/502] lib/string/format/num.lua
test [1] from the LuaJIT-test suite.

[1]: https://www.exploringbinary.com/incorrect-floating-point-to-decimal-conversions/

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#8825

Reviewed-by: Maxim Kokryashkin <m.kokryashkin@tarantool.org>
Reviewed-by: Sergey Bronnikov <sergeyb@tarantool.org>
Signed-off-by: Igor Munkin <imun@tarantool.org>

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