add build options

[mopemope]

Added optimal build options using cargo wizard.

[zbelial]

Hi @mopemope , thanks.
I have a question about panic = "abort" though, if we set this, I'm not sure whether a panic will make Emacs exit or not? If it will, we may lose the chance to see what causes panic, right?

[mopemope]

Hi. @zbelial
Yes, you're right.
I'll fix that since it's set to abort for speed.

[zbelial]

Thanks.
I just merged. But I'm thinking about deleting the lld flag later for these reasons:

1. Users have to install lld, and
2. I read from somewhere that lld is not fully supported yet. Here is a issue from rust-lang.
3. It just makes linking faster, so IMO it's not that important as the other changes.