docs: clarify that LLVM codegen is not available in unregisterised mode
The current docs are misleading and suggest that it is possible to use
LLVM codegen from an unregisterised build. This is not the case;
attempting to pass -fllvm
to an unregisterised build warns:
when making flags consistent: warning:
Target platform uses unregisterised ABI, so compiling via C
and uses the C codegen anyway.