Cross language LTO

[NobodyXu]

IMO this feature could be benefit crates such as cargo-binstall, which wants to reduce size of the final binary and does not use feature like compression.

It would also improves performance by inlining small C functions across language boundaries.

To enable this feature, zstd-sys will first have to re-enable LTO.

With rust-lang/cc-rs#757 merged, I think now lto support detection should be more robust and could be re-enabled.

Or we could enable lto only if feature thin is enabled, minimize people affected by this while experimenting with lto.

[NobodyXu]

#198 re-enabled LTO behind feature flags thin-lto and fat-lto.

[NobodyXu]

cargo-bins/cargo-binstall#817 enabled cross-lang-fat-lto in cargo-binstall and is able to reduce >50% of the final binary size on linux.

We haven't figured out for MacOS and Windows yet.