As of https://reviews.llvm.org/D110413, these no longer use the
unadjusted ABI (and use normal C ABI instead, passing i128
indirectly and returning it as <2 x i64>).
To support both LLVM 14 and older versions, rustc will expose a
"llvm14-builtins-abi" target feature, based on which
compiler-builtins can chose the appropriate ABI.
This is needed for rust-lang/rust#93577.