host: add support for OpenBSD/armv7

[geowiwi]

$ go version
go version go1.17 openbsd/arm
$ go test ./...                       
?       github.com/shirou/gopsutil/v3   [no test files]
ok      github.com/shirou/gopsutil/v3/cpu       0.232s
ok      github.com/shirou/gopsutil/v3/disk      0.062s
ok      github.com/shirou/gopsutil/v3/docker    0.065s       
ok      github.com/shirou/gopsutil/v3/host      0.085s
ok      github.com/shirou/gopsutil/v3/internal/common   0.109s       
ok      github.com/shirou/gopsutil/v3/load      0.109s
ok      github.com/shirou/gopsutil/v3/mem       0.126s
ok      github.com/shirou/gopsutil/v3/net       0.223s
ok      github.com/shirou/gopsutil/v3/process   11.309s

[shirou]

I haven't confirm it works or not, but seems awesome. Thank you so much!