ENH: hard-code finfo parameters for known types (#8504)

* ENH: hard-code finfo parameters for known types

Hard-code the MachAr parameters for float64, float32, 80-bit extended
precision, to save time, and provide skeleton for more difficult types
such as the double double on PPC; see
https://github.com/numpy/numpy/issues/2669

* ENH: add PPC long double finfo

Add parameters for PPC long double, fixing long-standing bug for finfo
on PPC.

* BF: use Python floats for float64 finfo

For some reason (garrgh) np.exp2 with float64 gives a different answer on
Windows than it does on other platforms; use Python floating point
calculations instead, which do appear to be consistent.

* DOC: add release notes for finfo fixes

Add release note describing fixes for PPC long double ``finfo``.

* RF: try using byte string to identify floats

From suggestion by Chuck, and
https//perl5.git.perl.org/perl.git/blob/3118d7d684b56cbeb702af874f4326683c45f045:/Configure

* TST: add tests for reasonable finfo parameters

Check that finfo returns somewhat reasonable parameters for all floating
point types.

* RF: add warning for not-recognized float type

Warn if we don't have a signature for the float type.

* NF: add IEEE float128 type signature

Still needs test on platform with IEEE float128, to check MachAr-like
parameters are correct.

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