Respect --llvm-opts when performing --llvm-lto=2

The changes the `--llvm-lto=2` pass to use any value of `--llvm-opts` specified in the command-line, rather than always using `-O3`.  I've found it useful to use e.g. `-Os` for this pass, to get some additional optimization without blowing up the code size.

AFAICT it will still default to `-O3` if `--llvm-opts` is not specified on the command-line.