Handle windows-on-arm ARM64 host arch in cmake

It seems CMake doesn't help normalize anything in this regard.

Author: alexcrichton

cmake/ba-download.cmake

@@ -9,6 +9,8 @@ function(ba_download target repo version)
     set(arch "aarch64")
   elseif (CMAKE_HOST_SYSTEM_PROCESSOR STREQUAL "AMD64")
     set(arch "x86_64")
+  elseif (CMAKE_HOST_SYSTEM_PROCESSOR STREQUAL "ARM64")
+    set(arch "aarch64")
   else()
     set(arch "UNKNOWN_ARCH")
     message(WARNING "Unsupported architecture ${CMAKE_HOST_SYSTEM_PROCESSOR} for ${target}")