Binaries for Windows on ARM

MSYS2/CLANG64 & CLANG64ARM environments support cross-compilation on 
Windows for Windows on ARM. I have just uploaded a first testing binary 
of gnuplot for Windows on ARM to the "testing" folder in the files 
section on SF (indicated by -woa). Please note that this is **completely 
untested** as I have no machine to test on. This version has no support 
(yet) for wxt, qt, gd terminals, and does not provide functions which 
rely on the openspecfun or cerf libraries and does not come with an 
installer. The 7zip-compressed archive should include all necessary 
DLLs, though. So far no ARM-specific changes were necessary.

Any feedback is very much appreciated, either via the list or at 
https://sourceforge.net/p/gnuplot/feature-requests/582/

   Bastian