Support MSYS2's CLANGARM64 environment on ARM64 Windows (crystal-lang…

…#15159)

Tested using a Windows VM on an Apple M2 host.

The instructions we use in our MinGW-w64 CI workflow will just work by simply replacing `-ucrt-` in the MSYS2 package names with `-clang-aarch64-`. Requires crystal-lang#15155 on both the cross-compilation host and the target.

src/lib_c/aarch64-windows-gnu

@@ -0,0 +1 @@
+x86_64-windows-msvc