Integrate RISC-V support #6

Job	Run time
build-native-gcc	4m 29s
build-native-llvm	5m 24s
test-native-gcc	4m 44s
test-native-llvm	4m 46s
build-aarch64-gcc	6m 17s
build-armhf-gcc	1m 11s
build-ppc64el-gcc	2m 10s
build-s390x-gcc	1m 47s
build-aarch64-llvm	4m 3s
build-armhf-llvm	2m 47s
build-ppc64el-llvm	4m 54s
build-s390x-llvm	9m 24s
build-riscv64-llvm	2m 6s
test-${{ matrix.arch }}-${{ matrix.compiler }} (qemu_cpu: "${{ matrix.qemu_cpu }}")	1s
	54m 3s

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