this commit cannot easily be squashed, since there is already a
PR based on the previous commit.

…, r=jieyouxu

migrate fmt-write-bloat to rmake

try-job: aarch64-apple
try-job: x86_64-gnu-llvm-18
try-job: dist-x86_64-linux

…riddle

nested aux-build in tests/rustdoc/ tests

* Fixes bug that prevented using nested aux-build in `tests/rustdoc/` tests. Before, `fn document` and the auxiliary builder disagreed about where to find the nested aux-build source file (`auxiliary/auxiliary/aux.rs` vs `auxiliary/aux.rs`), preventing them from building. Picked the latter in line with other builders in compiletest.
* Adds `//@ doc-flags` header, which forwards flags to rustdoc and not rustc.
* Adds `//@ unique-doc-out-dir` header, which sets the --out-dir for the rustdoc invocation to a unique directory: `<root out dir>/docs/<test name>/doc`
* Changes working directory of the rustdoc invocation to the root out directory (common among all aux-builds). Prior art: exec_compiled_test in runtest.rs
* Adds tests that use nested aux builds and new headers

These changes provide useful capabilities for writing rustdoc tests on their own. They are also needed to test the implementation for the [mergable-rustdoc-cross-crate-info](rust-lang/rfcs#3662) RFC.

try-job: x86_64-msvc

…ouxu

Migrate `cross-lang-lto` `run-make` test to rmake

Part of rust-lang#121876 and the associated [Google Summer of Code project](https://blog.rust-lang.org/2024/05/01/gsoc-2024-selected-projects.html).

Please try:

try-job: x86_64-msvc
try-job: i686-mingw
try-job: x86_64-mingw
try-job: armhf-gnu
try-job: test-various
try-job: aarch64-apple
try-job: x86_64-gnu-llvm-18

…jieyouxu

Migrate `link-cfg` and `rustdoc-default-output` `run-make` tests to rmake

Part of rust-lang#121876 and the associated [Google Summer of Code project](https://blog.rust-lang.org/2024/05/01/gsoc-2024-selected-projects.html).

try-job: aarch64-apple
try-job: x86_64-msvc
try-job: x86_64-mingw
try-job: x86_64-gnu-llvm-18
try-job: i686-msvc