buildx fix type convert compile error;
the type of the "s.Rdev" is 32bit on mip64el, and it cannot convert 'uint32' to 'uint64' implicitly with programming in GO language;
convert the "s.Rdev" type to 'uint64' explicitly;
Currently the user can type `docker buildx` to use this tool.
This patch allows the user to install buildx as a `docker builder` alias.
As an additional benefit, this allows the regular `docker build` to hook
into `buildx build`.
Note that the install and uninstall commands are currently hidden.
Signed-off-by: Tibor Vass <tibor@docker.com>
XiaodongLoong