There used to be a list of all the llvm ports and the status. The x86 was the only compiler
that was a "full port".
We are preparing to add out native linux compiler to the official build bots.
Are there various official "gating" criteria for different levels of llvm "doneness" so to speak?
There is a matrix I see in http://llvm.org/releases/3.0/docs/CodeGenerator.html#targetfeatures
which seems to be old. For example, I would definitely consider the MIPS port to be reliable and other
things like .o writing are definitely in there.
The various current llvm build bots seem to do different levels of testing.
While some platforms have very mature code and are generally reliable, we do not list them as officially supported because they don't go through the official release testing that X86 goes through. The official release testing is a fairly lengthy testing process (usually about a month) where we test clang/LLVM before we are confident that it can be called an official release. This involves running the full testsuite and making sure that there are no regressions from the previous release, and having the community compile their projects with the compiler and reporting any issues.
We don't say that we support anything other that X86 because we don't have resources -- both testers and equipment -- to run an official release on them. If we do have the resources, then we would be glad to list them as officially supported. The caveat being that testing is a fairly extensive commitment, but is very very welcome. 
-bw