Include entire flag in `optarch` (instead of excluding the initial dash) and maybe make it only per-compiler

[Micket]

The exclusion of the initial - from optarch strings is a confusing historical mistake. It looks especially weird when multiple flags are used, as it just append the initial -, so one ends up with stuff like

optarch = "march=x86-64 -mtune=generic"

which is just odd. If we also try to expand this into other compilers, Java, Rust, Go etc, we'd end up with things like C target-cpu=generic for Rust etc, which really just looks really strange.

At the same time, looking at multiple different compilers nvhpc vs gcc vs intel which don't have a unified set of flags (as well as more languages like Rust and Go where flags are completely different per compiler), we really don't have any chance of specifying general optarch; only the per-compiler version makes can work in general.

[boegel]

@Micket fixed with #4698, so can be closed?