Using the excellent llvm-mingw project [1] by @mstorsjo to implement
ARM64 support for curl-for-win builds.

ARM64 requires UCRT (this will not change) and clang (this may change
when GCC toolchains get ready for it.)

One small snag is OpenSSL, which doesn't have a native way for ARM64
support with mingw, so we use its `mingw64` mode with `no-asm` as a
workaround, which will affect performance.

ARM64 builds are not yet enabled in the CI machine. This will happen
in future commits.

For now this needs downloading llvm-mingw as a binary on each platform
and pointing the `CW_LLVM_MINGW_PATH` variable to it. llvm-mingw will
only be used for ARM64 targets for now, making versions listed in the
manifest a bit confusing.

Short of a CPU+VM or real machine to run ARM64 Windows apps, I have no
way to tell if the resulting binaries are correct or run at all. But
they for sure link cleanly.

Applied a pending PR to support ARM64 with Makefile.m32: [2]. Plus
an earlier patch to make it apply cleanly [3].

[1] https://github.com/mstorsjo/llvm-mingw
[2] curl/curl#9092
[3] curl/curl@c7f53b7

Test build with binaries here: [0]

Using the excellent llvm-mingw project [1] by @mstorsjo to implement
ARM64 support for curl-for-win builds.

ARM64 requires UCRT (this will not change) and clang (this may change
when GCC toolchains get ready for it.)

llvm-mingw is automatically downloaded, installed and used if
`CW_DL_LLVM_MINGW` is set. Existing llvm-mingw is used if envvar
`CW_LLVM_MINGW_PATH` is pointing to its directory. llvm-mingw is only
used for ARM64 targets for now. To make versions listed in the global
manifest `build.txt` less confusing, ARM64 tools are marked as such.

One small snag is OpenSSL, which doesn't have a native way for ARM64
support with mingw, so we use a slightly modified `mingw64` mode with
`no-asm` as a workaround, which affects performance. LibreSSL builds fine
out of the box.

Short of a CPU+VM or real machine to run ARM64 Windows apps, I have no
way to tell if the resulting binaries are correct or run at all. But,
they link cleanly. I also have not tested a build on Windows, only on
Linux and macOS.

ARM64 builds are now enabled for CI builds. One potential problem is
builds taking very long now. With x64+x86 using Makefile.m32, a typical
time was 25-30 minutes. With x64+x86+a64 using CMake, it's 50 minutes,
which may hit CI limits. Can be mitigated by switching back to faster
Makefile.m32 builds from CMake for curl.

Applied a pending PR to support ARM64 with Makefile.m32: [2]. Plus an
earlier patch to make that apply cleanly [3].

[0] https://ci.appveyor.com/project/curlorg/curl-for-win/builds/44062838
[1] https://github.com/mstorsjo/llvm-mingw
[2] curl/curl#9092
[3] curl/curl@c7f53b7