Refector Gitian Descriptors to build tools seprately (and preserve them)
The fixes to #9084 (closed) and #9828 (closed) (and soon #9829 (closed)) have us building more and more of the toolchain and supporting tools from Gitian.
We should make a single, separate descriptor for all of these tool outputs (we only need to build them for Linux anyway), and for any future toolchain components we build as part of our efforts to build from multiple Gitian guest OS distributions.
The makefiles should also be updated to treat these outputs specially (don't erase them during 'make clean', and find some way to rebuild them if their version in the 'versions' file changes).