cmake(): allow setting HOST_CPU for cross-compilation

[dennisameling]

Related to #3107

As reported by @rimrul in #3083 (comment), git version --build-options returns the wrong CPU when building Git for ARM64 Windows.

Git's regular Makefile mentions that HOST_CPU should be defined when cross-compiling Git:

git/Makefile

Lines 438 to 439 in 37796bc

	# When cross-compiling, define HOST_CPU as the canonical name of the CPU on
	# which the built Git will run (for instance "x86_64").

This is then used to set the GIT_HOST_CPU variable when compiling Git:

git/Makefile

Lines 1337 to 1341 in 37796bc

	ifeq (,$(HOST_CPU))
	BASIC_CFLAGS += -DGIT_HOST_CPU="\"$(firstword $(subst -, ,$(uname_M)))\""
	else
	BASIC_CFLAGS += -DGIT_HOST_CPU="\"$(HOST_CPU)\""
	endif

Then, when the user runs git version --build-options, it returns that value:

git/help.c

Line 658 in 37796bc

strbuf_addf(buf, "cpu: %s\n", GIT_HOST_CPU);

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

I split this into 2 separate commits, since .github/git-artifacts.yml is Git for Windows-specific and therefore can't be upstreamed.

Artifacts run: https://github.com/dennisameling/git/actions/runs/1044960096 (note that this only applies to Git for Windows arm64 since the other architectures are still built using the original Makefile)

[dscho]

No worries about the macOS failures, I opened Homebrew/homebrew-cask#108819 which will hopefully soon be merged (and should fix this issue).

[dscho]

Thank you @dennisameling!