move -Z lto to -C lto

[thestinger]

It doesn't make sense as a debug option. This was where experimental options used to go but the documentation now just says they're debugging flags.

[alexcrichton]

I initially didn't do this because I wasn't convinced that we're satisfied without LTO works right now, so I figured we should reserve the right to change it. That being said, we're also not sure that -C link-args will always be supported, yet I went ahead and moved it. It's probably ok to move.

[reem]

Triage: This should probably be moved as it's not a debugging flag.