refactor mingw-w64 self-contained detection

[mati865]

Split out from #147536 (comment)

This change decouples detect_self_contained_mingw() from the linker determined by linker_and_flavor(), allowing for better control over the process.

There should only one user observable change from this PR. Passing -Clinker=<linker> when is absent from PATH, will result in an error regarding inability to find <linker>, unless -Clink-self-contained is also provided.
Previously, -Clinker=<linker> allowed to call any executable shipped with Rust, including rust-objcopy, wasm-component-ld.

r? @petrochenkov

[mati865]

@bors try jobs=dist-x86_64-windows-gnu

[rust-bors]

💔 Test for 430b82e failed: CI. Failed job:

• Calculate job matrix (web logs, extended logs)

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling ureq v3.0.8
   Compiling citool v0.1.0 (/home/runner/work/rust/rust/src/ci/citool)
    Finished `dev` profile [unoptimized] target(s) in 21.03s
     Running `target/debug/citool calculate-job-matrix`
Run type: TryJob { job_patterns: Some(["dist-x86_64-windows-gnu"]) }
Error: Failed to calculate job matrix

Caused by:
    Patterns `dist-x86_64-windows-gnu` did not match any auto jobs
##[error]Process completed with exit code 1.

[mati865]

@bors try jobs=dist-x86_64-mingw

[rust-bors]

☀️ Try build successful (CI)
Build commit: 104fcc2 (104fcc2a417e55791a997db8b9c3fc0b8885d4d9, parent: e22dab387f6b4f6a87dfc54ac2f6013dddb41e68)

[mati865]

There should only one user observable change from this PR. Passing -Clinker=<linker> when is absent from PATH, will result in an error regarding inability to find <linker>, unless -Clink-self-contained is also provided.
Previously, -Clinker=<linker> allowed to call any executable shipped with Rust, including rust-objcopy, wasm-component-ld.

@petrochenkov
IMO, forbidding to call any Rust shipped binary is clearly a benefit here, but what should we do about -Clinker=x86_64-w64-mingw32-gcc and -Clinker=ld no longer using self-contained mode?
The first one could be handled by an additional sess.opts.cg.linker == sess.target.linker condition (not like that because of different types, but you get the idea). Retaining -Clinker=ld as self-contained without -Clink-self-contained, would unfortunately require hardcoding the name.