[LTO] Set codegen units to 1

[ksolana]

Problem

@LucasSte suggested we should experiment thinlto builds by setting codegen units to 1
As per Source: https://nnethercote.github.io/perf-book/build-configuration.html#codegen-units

he Rust compiler splits crates into multiple [codegen units](https://doc.rust-lang.org/cargo/reference/profiles.html#codegen-units) to parallelize (and thus speed up) compilation. However, this might cause it to miss some potential optimizations. You may be able to improve runtime speed and reduce binary size, at the cost of increased compile times, by setting the number of units to one. Add these lines to the Cargo.toml file:

Proposed Solution

Add codegen-units = 1 to the build setting.