README: add cross-compile #299

Job	Run time
build-native-ubuntu-24.04-gcc	4m 10s
build-native-ubuntu-24.04-llvm	3m 13s
build-native-ah-ubuntu_22_04-c7g_2x-50-gcc	1m 59s
build-native-ah-ubuntu_22_04-c7g_2x-50-llvm	2m 31s
test-native-ubuntu-24.04-gcc	8m 20s
build-armhf-gcc	4m 43s
test-native-ubuntu-24.04-llvm	8m 9s
build-armhf-llvm	5m 27s
test-native-ah-ubuntu_22_04-c7g_2x-50-gcc	2m 40s
build-ppc64el-gcc	5m 1s
test-native-ah-ubuntu_22_04-c7g_2x-50-llvm	2m 32s
build-ppc64el-llvm	6m 43s
build-s390x-gcc	5m 25s
build-s390x-llvm	6m 11s
build-riscv64-gcc	5m 59s
build-riscv64-llvm	6m 41s
test-${{ matrix.arch }}-${{ matrix.compiler }} (qemu_cpu: "${{ matrix.qemu_cpu }}")	0s
	1h 19m 44s

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