Build openblas with ucrt mingw

[mattip]

Replaces #75 since I don't have push permissions to that repo.

Fixes #68 fixes #74

I don't understand some of the differences between build_openblas.ps1 and build_openblas.sh. Some of the variable names have changed, there is some renaming that is different, and it seems the zip step at the end is missing.

[matthew-brett]

Yes - sorry - the .ps1 file was just me working it out as I went. It really needs a rewrite, and it will probably work as a refactor of the .sh script. Yes, it does need the -ucrt toolchain - Scipy is using rtools-4 from choco - I believe.

[carlkl]

AFAIK OpenBLAS still uses long double for intermediate calculations. i.e.: interface/zrotg.c
So using the $long_double="mlong-double-64" flag on Windows is not recommended without patching OpenBLAS.
See i.e. OpenMathLib/OpenBLAS#1711 and OpenMathLib/OpenBLAS#1270. See also this comment: OpenMathLib/OpenBLAS#1270 (comment)
The non-standard toolchain called mingwpy was at this time a mingw-w64 toolchain patched to use "mlong-double-64" as default.

[mattip]

Closing in favor of #82