Add support to ARM and ARM64 Windows targets.

[woachk]

Related to #558.

ARM and ARM64 Windows targets depend on LLVM-MinGW https://github.com/mstorsjo/llvm-mingw, and aren't usable with a binutils + GCC based toolchain.

[kleisauke]

fwiw, I've created a MXE-plugin that uses https://github.com/mstorsjo/llvm-mingw as toolchain, see:
https://github.com/libvips/build-win64-mxe/tree/master/build/plugins/llvm-mingw

I'm considering making a PR to integrate it into MXE as well. Is there any interest in this?

(Still need to test the aarch64-w64-mingw32 target but I don't have the necessary hardware to test it on, unfortunately)

[lunixbochs]

@kleisauke I am in favor of an in-tree LLVM plugin, I keep running into issues with GCC

[boswelja]

Any updates on this?

[tonytheodore]

Adding clang is the next thing on my list now, gcc doesn't yet support newer Macs with M1 chips.

The repo above looks very promising, thanks @kleisauke.

[markghayden]

Is there any progress on this? The Windows ARM-based Surface tablets are becoming more common and we'd like to compile for them.

[kalibera]

I've extended the libvips plugin slightly and adapted several MXE packages to build for Windwos/aarch64 using llvm-mingw. I've also added a plugin for building native LLVM compilers. I've included flang-new. This is all quick and dirty at this point, but I am sharing in case it was useful in creating a proper LLVM support in MXE. The code is under https://svn.r-project.org/R-dev-web/trunk/WindowsBuilds/winutf8/ucrt3/toolchain_libs/mxe (version 5667 should be most stable at this point). It is sufficient to build R (natively) including recommended packages, so that it passes its own tests. A number of other MXE packages can be built, which are needed for CRAN contributed packages, but that hasn't been tested, yet. This is in scope of the R project (more in https://blog.r-project.org/2023/08/23/will-r-work-on-64-bit-arm-windows/index.html). Of course, it would be great to have proper LLVM support in upstream MXE, not only to support Windows/aarch64, but say also to be able to support other LLVM compilers (e.g. rust).