Support RISCV64

[qdm12]

There is the dependency github.com/prometheus/procfs which added support for riscv64 in v0.6.0 (prometheus/procfs@910e685)
Currently trying to build for riscv64:

GOARCH=riscv64 go build  ./cmd/buildx

will fail with

/go/pkg/mod/github.com/prometheus/procfs@v0.1.3/cpuinfo.go:71:9: undefined: parseCPUInfo

buildx probably depends on many dependencies using versions before v0.6.0 so it's more of a long shot than something that can be fixed in a single PR I believe. Although

go get github.com/prometheus/procfs@v0.6.0
go mod tidy

Fixes the build, but probably in a risky-ish way I guess. Not a priority issue at all, but I thought it would be useful to track it.

[tonistiigi]

Procfs was updated in #565 d40a608#diff-b318e1e6bbcc59892bb6b4298564b051c6f6d5d848878d14aab856509ff0625cR333

Added CI validation in #652

[qdm12]

Ah indeed, sorry for the mistake, I was using the latest release v0.5.1. Any plan to make another release sometime by the way? Thanks!