Valide platform on loading config.json
Valide platform on loading config.json

run an arm64 image on an amd64 platform, it will failed with
````
panic: standard_init_linux.go:175: exec user process caused "exec format error" [recovered]
        panic: standard_init_linux.go:175: exec user process caused "exec format error"

goroutine 1 [running, locked to thread]:
panic(0x7e7e40, 0xc820124380)
        /usr/local/go/src/runtime/panic.go:481 +0x3e6
github.com/urfave/cli.HandleAction.func1(0xc8200c52f8)
        /home/lei/opencontainers/runc/Godeps/_workspace/src/github.com/urfave/cli/app.go:478 +0x38e
panic(0x7e7e40, 0xc820124380)
        /usr/local/go/src/runtime/panic.go:443 +0x4e9
github.com/opencontainers/runc/libcontainer.(*LinuxFactory).StartInitialization.func1(0xc8200c4c08, 0xc8200220a0, 0xc8200c4d18)
        /home/lei/opencontainers/runc/Godeps/_workspace/src/github.com/opencontainers/runc/libcontainer/factory_linux.go:259 +0x136
github.com/opencontainers/runc/libcontainer.(*LinuxFactory).StartInitialization(0xc82006c780, 0x7fb9124733f8, 0xc820124380)
        /home/lei/opencontainers/runc/Godeps/_workspace/src/github.com/opencontainers/runc/libcontainer/factory_linux.go:277 +0x5b1
main.glob.func8(0xc820090780, 0x0, 0x0)
        /home/lei/opencontainers/runc/main_unix.go:26 +0x68
reflect.Value.call(0x74ca00, 0x8fda40, 0x13, 0x844470, 0x4, 0xc8200c5278, 0x1, 0x1, 0x0, 0x0, ...)
        /usr/local/go/src/reflect/value.go:435 +0x120d
reflect.Value.Call(0x74ca00, 0x8fda40, 0x13, 0xc8200c5278, 0x1, 0x1, 0x0, 0x0, 0x0)
        /usr/local/go/src/reflect/value.go:303 +0xb1
github.com/urfave/cli.HandleAction(0x74ca00, 0x8fda40, 0xc820090780, 0x0, 0x0)
        /home/lei/opencontainers/runc/Godeps/_workspace/src/github.com/urfave/cli/app.go:487 +0x2ee
github.com/urfave/cli.Command.Run(0x847330, 0x4, 0x0, 0x0, 0x0, 0x0, 0x0, 0x8dcec0, 0x51, 0x0, ...)
        /home/lei/opencontainers/runc/Godeps/_workspace/src/github.com/urfave/cli/command.go:191 +0xfec
github.com/urfave/cli.(*App).Run(0xc820001980, 0xc82000a100, 0x2, 0x2, 0x0, 0x0)
        /home/lei/opencontainers/runc/Godeps/_workspace/src/github.com/urfave/cli/app.go:240 +0xaa4
main.main()
        /home/lei/opencontainers/runc/main.go:137 +0xe24

````
instead of throwing panic on execing the container process, we should
throw the platform mismatch at the very beginning, it's much more
clear and can tell user what's wrong.

Signed-off-by: Lei Jitang <9ac444d2b5df3db1f31aa1c6462ac8e9e2bde241@huawei.com>

sunyuan3/runc,stefanberger/runc,rhatdan/runc,estesp/runc,mheon/runc,xiaochenshen/runc,opencontainers/runc,laijs/runc,jhowardmsft/runc,stefanberger/runc,docker/runc,dqminh/runc,coolljt0725/runc,rhatdan/runc,estesp/runc,avagin/runc,estesp/runc,shishir-a412ed/runc,vbatts/runc,maxamillion/runc,cyphar/runc,justincormack/runc,kolyshkin/runc,mYmNeo/runc,shishir-a412ed/runc,mikebrow/runc,rhatdan/runc,adrianreber/runc,crosbymichael/runc,mYmNeo/runc,mikebrow/runc,jhowardmsft/runc,cloudfoundry-incubator/runc,hqhq/runc,kolyshkin/runc,adrianreber/runc,justincormack/runc,mikebrow/runc,shishir-a412ed/runc,docker/runc,justincormack/runc,adrianreber/runc,sunyuan3/runc,opencontainers/runc,avagin/runc,kolyshkin/runc,crosbymichael/runc,vbatts/runc,dqminh/runc,rajasec/runc,cyphar/runc,dqminh/runc,cloudfoundry-incubator/runc,WeiZhang555/runc,cyphar/runc,rhatdan/runc,maxamillion/runc,xiaochenshen/runc,wking/runc,opencontainers/runc,wking/runc,mheon/runc,WeiZhang555/runc,mheon/runc,sunyuan3/runc,laijs/runc,maxamillion/runc,hqhq/runc,rajasec/runc,dqminh/runc,hqhq/runc,opencontainers/runc,mYmNeo/runc,coolljt0725/runc,cloudfoundry-incubator/runc,vbatts/runc,xiaochenshen/runc,wking/runc,WeiZhang555/runc,docker/runc,stefanberger/runc,avagin/runc,laijs/runc,kolyshkin/runc,jhowardmsft/runc,crosbymichael/runc,coolljt0725/runc,rajasec/runc