Add new source component that includes GPL-licensed source

[Kobzol]

To unblock rustup distribution of cg_gcc (https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Safe.20rustup.20distribution.20of.20rustc_codegen_gcc/with/560644441).

CC @joshtriplett

r? @Mark-Simulacrum

[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

cargo:warning=Compiler family detection failed due to error: ToolNotFound: failed to find tool "x86_64-linux-gnu-g++": No such file or directory (os error 2)
cargo:warning=Compiler family detection failed due to error: ToolNotFound: failed to find tool "x86_64-linux-gnu-g++": No such file or directory (os error 2)
cargo:warning=Compiler family detection failed due to error: ToolNotFound: failed to find tool "x86_64-linux-gnu-g++": No such file or directory (os error 2)
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ Snapshot Summary ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Snapshot file: src/bootstrap/src/core/builder/cli_paths/snapshots/x_dist.snap
Snapshot: x_dist
Source: src/bootstrap/src/core/builder/cli_paths/tests.rs:68
────────────────────────────────────────────────────────────────────────────────
Expression: dist
────────────────────────────────────────────────────────────────────────────────
-old snapshot
+new results
────────────┬───────────────────────────────────────────────────────────────────
   23    23 │     - Set({dist::rust-dev})
   24    24 │ [Dist] dist::PlainSourceTarball
   25    25 │     targets: [x86_64-unknown-linux-gnu]
   26    26 │     - Set({dist::rustc-src})
         27 │+[Dist] dist::PlainSourceTarballGpl
         28 │+    targets: [x86_64-unknown-linux-gnu]
         29 │+    - Set({dist::rustc-src-gpl})
   27    30 │ [Dist] dist::ReproducibleArtifacts
   28    31 │     targets: [x86_64-unknown-linux-gnu]
   29    32 │     - Set({dist::reproducible-artifacts})
────────────┴───────────────────────────────────────────────────────────────────
Stopped on the first failure. Run `cargo insta test` to run all snapshots.

thread 'core::builder::cli_paths::tests::x_dist' (7029) panicked at /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/insta-1.43.1/src/runtime.rs:679:13:
snapshot assertion for 'x_dist' failed in line 68
stack backtrace:
   0: __rustc::rust_begin_unwind
             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/std/src/panicking.rs:698:5
   1: core::panicking::panic_fmt
             at /rustc/3b4dd9bf1410f8da6329baa36ce5e37673cbbd1f/library/core/src/panicking.rs:80:14
   2: <insta::runtime::SnapshotAssertionContext>::finalize
             at /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/insta-1.43.1/src/runtime.rs:679:13
   3: insta::runtime::assert_snapshot
             at /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/insta-1.43.1/src/runtime.rs:848:13
   4: bootstrap::core::builder::cli_paths::tests::snapshot_test_inner::{closure#0}
             at ./src/core/builder/cli_paths/tests.rs:68:9
   5: <insta::settings::Settings>::bind::<bootstrap::core::builder::cli_paths::tests::snapshot_test_inner::{closure#0}, ()>
             at /cargo/registry/src/index.crates.io-1949cf8c6b5b557f/insta-1.43.1/src/settings.rs:509:9
   6: bootstrap::core::builder::cli_paths::tests::snapshot_test_inner
             at ./src/core/builder/cli_paths/tests.rs:67:14
   7: bootstrap::core::builder::cli_paths::tests::x_dist
             at ./src/core/builder/cli_paths/tests.rs:110:17
   8: bootstrap::core::builder::cli_paths::tests::x_dist::{closure#0}