cmake(): allow setting HOST_CPU for cross-compilation

Git's regular Makefile mentions that HOST_CPU should be defined when cross-compiling Git: https://github.com/git-for-windows/git/blob/37796bca76ef4180c39ee508ca3e42c0777ba444/Makefile#L438-L439

This is then used to set the GIT_HOST_CPU variable when compiling Git: https://github.com/git-for-windows/git/blob/37796bca76ef4180c39ee508ca3e42c0777ba444/Makefile#L1337-L1341

Then, when the user runs `git version --build-options`, it returns that value: https://github.com/git-for-windows/git/blob/37796bca76ef4180c39ee508ca3e42c0777ba444/help.c#L658

This commit adds the same functionality to the CMake configuration. Users can now set -DHOST_CPU= to set the target architecture.

Signed-off-by: Dennis Ameling <dennis@dennisameling.com>

.github/workflows/main.yml

@@ -197,7 +197,7 @@ jobs:
       shell: bash
       run: |
         cmake `pwd`/contrib/buildsystems/ -DCMAKE_PREFIX_PATH=`pwd`/compat/vcbuild/vcpkg/installed/${{ matrix.arch }}-windows \
-        -DNO_GETTEXT=YesPlease -DPERL_TESTS=OFF -DPYTHON_TESTS=OFF -DCURL_NO_CURL_CMAKE=ON -DCMAKE_GENERATOR_PLATFORM=${{ matrix.arch }} -DVCPKG_ARCH=${{ matrix.arch }}-windows
+        -DNO_GETTEXT=YesPlease -DPERL_TESTS=OFF -DPYTHON_TESTS=OFF -DCURL_NO_CURL_CMAKE=ON -DCMAKE_GENERATOR_PLATFORM=${{ matrix.arch }} -DVCPKG_ARCH=${{ matrix.arch }}-windows -DHOST_CPU=${{ matrix.arch }}
     - name: MSBuild
       run: msbuild git.sln -property:Configuration=Release -property:Platform=${{ matrix.arch }} -maxCpuCount:4 -property:PlatformToolset=v142
     - name: bundle artifact tar

contrib/buildsystems/CMakeLists.txt

@@ -223,7 +223,14 @@ endif()
 
 #default behaviour
 include_directories(${CMAKE_SOURCE_DIR})
-add_compile_definitions(GIT_HOST_CPU="${CMAKE_SYSTEM_PROCESSOR}")
+
+# When cross-compiling, define HOST_CPU as the canonical name of the CPU on
+# which the built Git will run (for instance "x86_64").
+if(NOT HOST_CPU)
+	add_compile_definitions(GIT_HOST_CPU="${CMAKE_SYSTEM_PROCESSOR}")
+else()
+	add_compile_definitions(GIT_HOST_CPU="${HOST_CPU}")
+endif()
 add_compile_definitions(SHA256_BLK INTERNAL_QSORT RUNTIME_PREFIX)
 add_compile_definitions(NO_OPENSSL SHA1_DC SHA1DC_NO_STANDARD_INCLUDES
 			SHA1DC_INIT_SAFE_HASH_DEFAULT=0