Enable libsleefdft on riscv64 #37

Job	Run time
build-native-gcc	3m 13s
build-native-llvm	3m 42s
test-native-gcc	9m 44s
build-aarch64-gcc	5m 4s
build-aarch64-llvm	2m 57s
test-native-llvm	7m 12s
build-armhf-gcc	1m 38s
build-armhf-llvm	2m 34s
build-ppc64el-gcc	3m 57s
build-ppc64el-llvm	4m 8s
build-s390x-gcc	2m 15s
build-s390x-llvm	5m 33s
build-riscv64-llvm	1m 42s
test-${{ matrix.arch }}-${{ matrix.compiler }} (qemu_cpu: "${{ matrix.qemu_cpu }}")	0s
	53m 39s

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