Add riscv64 CI on GitHub Actions #69

Job	Run time
build-and-test	6m 23s
build-and-test-cross (aarch64, max,sve=off)	11m 51s
build-and-test-cross (aarch64, max,sve=on,sve128=on)	12m 18s
build-and-test-cross (aarch64, max,sve=on,sve256=on)	12m 16s
build-and-test-cross (aarch64, max,sve=on,sve512=on)	14m 2s
build-and-test-cross (armhf, arm, -arm-linux-gnueabihf)	2m 53s
build-and-test-cross (ppc64el, ppc64le, -powerpc64le-linux-gnu)	2m 55s
build-and-test-cross (s390x)	2m 34s
build-and-test-cross (riscv64, rv64,zba=true,zbb=true,zbs=true,v=false)	7m 9s
build-and-test-cross (riscv64, rv64,zba=true,zbb=true,zbs=true,v=true,vlen=128,elen=64,vext_spec=...	11m 52s
build-and-test-cross (riscv64, rv64,zba=true,zbb=true,zbs=true,v=true,vlen=256,elen=64,vext_spec=...	11m 55s
build-and-test-cross (riscv64, rv64,zba=true,zbb=true,zbs=true,v=true,vlen=512,elen=64,vext_spec=...	10m 36s
	1h 46m 44s

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