build: Add cross-build target

This builds both virtualization-amd64 and virtualization-arm64 for easier
cross-platform testing.
https://github.com/randall77/makefat can then be used to generate a universal
binary if needed.

example/linux/Makefile

@@ -5,6 +5,19 @@ all: build codesign
 codesign:
 	codesign --entitlements vz.entitlements -s - ./virtualization
 
-.PHONY: build
+.PHONY: build cross-build
 build:
 	go build -o virtualization .
+
+cross-build: virtualization-amd64 virtualization-arm64
+
+virtualization-amd64 virtualization-arm64: virtualization-%: force-build
+	CGO_ENABLED=1 GOOS=darwin GOARCH=$* go build -o $@ .
+
+# the go compiler is doing a good job at not rebuilding unchanged files
+# this phony target ensures out/vfkit-* are always considered out of date
+# and rebuilt. If the code was unchanged, go won't rebuild anything so that's
+# fast. Forcing the rebuild ensure we rebuild when needed, ie when the source code
+# changed, without adding explicit dependencies to the go files/go.mod
+.PHONY: force-build
+force-build:

example/macOS/Makefile

@@ -5,6 +5,19 @@ all: build codesign
 codesign:
 	codesign --entitlements vz.entitlements -s - ./virtualization
 
-.PHONY: build
+.PHONY: build cross-build
 build:
 	go build -o virtualization .
+
+cross-build: virtualization-amd64 virtualization-arm64
+
+virtualization-amd64 virtualization-arm64: virtualization-%: force-build
+	CGO_ENABLED=1 GOOS=darwin GOARCH=$* go build -o $@ .
+
+# the go compiler is doing a good job at not rebuilding unchanged files
+# this phony target ensures out/vfkit-* are always considered out of date
+# and rebuilt. If the code was unchanged, go won't rebuild anything so that's
+# fast. Forcing the rebuild ensure we rebuild when needed, ie when the source code
+# changed, without adding explicit dependencies to the go files/go.mod
+.PHONY: force-build
+force-build: