build: cargo: Set default release profile to use LTO

[jpds]

Linux distros use this target for their end-user packages, as such this needs to be the default for those users to take advantage of the performance benefits.

[plusvic]

I'm not convinced about making LTO the default because compilation is very slow, specially when used together with codegen-units = 1. The default release profile is good enough, and produces very fast code. The real benefit I see in using the release-lto profile is reducing the executable size.

I prefer using as the default release configuration whatever cargo choses as default. Anything that deviates from cargo's default, is a special profile used for specific purposes.