Rework `run-make-support`'s `target` to be a structured API

run-make tests often need to have target-specific behavior (or have a distinction between host / target). We should make support for that properly structured and not via substring matching. E.g. instead of the various is_windows_msvc and target helpers, introduce a structured target API:

fn host() -> TargetInfo;
fn target() -> Targetinfo;

#[non_exhaustive]
pub struct TargetInfo {
    pub target_tuple: String,
    pub arch: Arch, // possibly with a subarch too
    pub env: Env, // with a Env::None
    pub os: Os, // with a Os::None
    pub vendor: Vendor, // with a Vendor::None
    pub target_family: TargetFamily, // with a TargetFamily::None
}

Also, consider changing the run-make test <-> run_make_support <-> compiletest env vars to be prefixed as internal-only, then only "publicly" expose this to run-make tests.