RISC-V architecture not supported in bst master

[jjardon]

Error in freedesktop-sdk when trying to build with bst master (bst-1 is fine): https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/jobs/2012261945

[jjardon]

@gtristan @abderrahim can we please add this to https://github.com/apache/buildstream/milestone/3 ? fdsdk will not be able to build without this

[juergbi]

I've opened bazelbuild/remote-apis#227 to update the REAPI platform lexicon.

[jjardon]

Corresponding PR for buildstream for when above gets merged here: #1670

[juergbi]

Merged.