x/website: add linux/riscv64 downloads

[ernado]

Add linux/riscv64 to go.dev/dl to downloads table under "Other Ports" (i.e. like go1.20.2.linux-ppc64le.tar.gz).

Here:

This is required to add riscv64 to Docker Official Image packaging for golang.

I've tried to search for repo to apply CL's, but failed (golang/build?).

[seankhliao]

As per our https://github.com/golang/go/wiki/PortingPolicy
linux/risc64 is a secondary port, the core team won't be publishing binary builds for this architecture.

See also #53383

[ernado]

@seankhliao linux/s390x is a secondary port too, same as all ports under "Other Ports" section, so port should not be first class to have binaries.

[ernado]

Oh, I see the porting policy now.

Removing secondary ports will break some docker official images, which is unfortunate.

It looks like there should be another process to publish binaries, I'll investigate.

[ernado]

Ref: #53862

[ernado]

I've created go-riscv/go that cross-compiles compiler for riscv64 in CI/CD.

There is a pre-compiled go distrubiton go1.20.2.linux-riscv64.tar.gz in latest release and also scripts for building it locally.

[ernado]

This was done as part of #53862.