llvm dev - Oct 2021 - 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 https://libcxx.llvm.org/BuildingLibcxx.html, 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?)

Thanks,
Tom
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