TIL. I didn't know clang supports this natively.

Clang is natively a cross-compiler. Pass in --sysroot and a corresponding valid sysroot tree for any micro architecture/platform (arm-eabi, macOS, Windows MSVC, PowerPC, Alpine Linux with musl, you name it) and Clang will happily retarget the binary to the correct target platform.

Apple has supported that ‘-arch’ option in their GCC/Clang since at least the PPC->Intel transition, maybe even earlier (PPC64? NeXT/OPENSTEP?)

Yes, since NEXTSTEP.

That would be NeXT, and their GCC fork.

I assume this is faster than doing two separate builds, because it can skip certain steps of the complier pipeline, and only the items that are arch specific (codegen, probably others) are done twice?

They can't really share anything since the preprocessor stage can be different.

How much work does clang have to do for this sort of thing (as opposed to llvm). Hypothetically could we start distributing programs in llvm ir, and compile that locally to ARM, x86, risc-v, or whatever else?

I mean, no, that’s silly, right? But it would be kind of neat…

llvm-ir is not architecture independent, even excluding issues like the C pre-processor.