When building for a baremetal triple an extraneous ".a" is appended to the clang_rt.builtins library. This causes the linker to fail to find the static library, as the flag passed to the linker is -lclang_rt.builtins-arm.a instead of -lclang_rt.builtins-arm.
ld.lld: error: unable to find library -lclang_rt.builtins-arm.a
Removing this extraneous '.a' produces the desired behavior and the link succeeds. Tests were updated to reflect the change.