Add support for Windows on ARM (#713)

Bring support for Windows on ARM64 to this fork of Git. We make use of a
custom 1ES hosted pool (hosted in Azure) that runs Windows 11 ARM64 on a
`B4pls v2` sized VM SKU. The image has the latest Git for Windows ARM64
installed, the GitHub Actions runner, along with PowerShell Core
(required for the GitHub Actions runner).

Requests with the `['self-hosted', '1ES.Pool=github-arm64-pool']` labels
will be directed to the 1ES hosted pool.

A few changes were made to the `build-git-installers.yml` workflow,
mainly to parameterise the architecture to allow for x86_64 and aarch64
builds, via a matrix strategy.

Find the end-to-end successful build of `build-git-installers.yml`
[here](https://github.com/microsoft/git/actions/runs/12358457268).

Author: mjcheetham

.github/workflows/clangarm64-build.yml

@@ -9,7 +9,7 @@ defaults:
 
 jobs:
   clang-build:
-    runs-on: [Windows, ARM64]
+    runs-on: ['self-hosted', '1ES.Pool=github-arm64-pool']
     env:
       NO_PERL: 1
     steps: