rustc_target: Move some target options from `Target` to `TargetOptions`
The only reason for `Target` to `TargetOptions` to be separate structures is that options in `TargetOptions` have reasonable defaults and options in `Target` don't.
(Otherwise all the options logically belong to a single `Target` struct.)
This PR moves a number of options with reasonable defaults from `Target` to `TargetOptions`, so they no longer needs to be specified explicitly for majority of the targets.
The move also allows to inherit the options from `rustc_target/src/spec/*_base.rs` files in a nicer way.
I didn't change any specific option values here.
The moved options are `target_c_int_width` (defaults to `"32"`), `target_endian` (defaults to `"little"`), `target_os` (defaults to `"none"`), `target_env` (defaults to `""`), `target_vendor` (defaults to `"unknown"`) and `linker_flavor` (defaults to `LinkerFlavor::Gcc`).
Next steps (in later PRs):
- Find a way to merge `TargetOptions` into `Target`
- If not, always access `TargetOptions` fields through `Deref` making it a part of `Target` at least logically (`session.target.target.options.foo` -> `session.target.target.foo`)
- ~Eliminate `session::config::Config` and use `Target` instead (`session.target.target.foo` -> `session.target.foo`)~ Done in https://github.com/rust-lang/rust/pull/77943.
- Avoid tautologies in option names (`target.target_os` -> `target.os`)
- Resolve _ https://github.com/rust-lang/rust/issues/77730 (rustc_target: The differences between `target_os = "none"` and `target_os = "unknown"`, and `target_vendor = "unknown"` and `target_vendor = ""` are unclear) noticed during implementation of this PR.
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