[libc-commits] [libc] [libc] Use `${libc_opt_high_flag}` instead of `-O3` (PR #123233)

[Nick Desaulniers via libc-commits]

nickdesaulniers wrote:

One thing to triple check is that when cross compiling (`-DLIBC_TARGET_TRIPLE=aarch64-linux-gnu`) and using lld (`-DLLVM_ENABLE_LLD=ON`), do we still pass `--target=aarch64-linux-gnu` to the linker?

https://github.com/llvm/llvm-project/pull/123233