build: add fedora_rv64 rootfs #38

Job	Run time
mkrootfs (canmv, yocto_rv32, rv64ilp32)	10m 5s
mkrootfs (canmv, yocto_rv32, rv64)	9m 58s
mkrootfs (canmv, fedora_rv32, rv64ilp32)	10m 48s
mkrootfs (canmv, fedora_rv32, rv64)	10m 53s
mkrootfs (canmv, fedora_rv64, rv64)	11m 10s
mkrootfs (evb, yocto_rv32, rv64ilp32)	10m 21s
mkrootfs (evb, yocto_rv32, rv64)	9m 57s
mkrootfs (evb, fedora_rv32, rv64ilp32)	10m 30s
mkrootfs (evb, fedora_rv32, rv64)	10m 56s
mkrootfs (evb, fedora_rv64, rv64)	11m 17s
	1h 45m 55s

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