Building libc++ for a RISCV bare target?

Is it possible to build libc++ without OS support, e.g. with code such as structured I/O or threads absent or replaced by non-functional stubs?

(based on, I had an optimistic try with -DLLVM_RUNTIME_TARGETS=riscv32-unknown-elf . That fails, but possibly some other set of control macros can achieve this?)