rustdoc: save target modifiers

`rustdoc` was filling a `target_modifiers` variable, but it was not
using the result.

In turn, that means that trying to use a dependency that set a target
modifier fails.

For instance, running:

```sh
RUSTC_BOOTSTRAP=1 rustc --edition=2024 --target=aarch64-unknown-none-softfloat --sysroot=/dev/null --emit=metadata -Zfixed-x18 --crate-type rlib --crate-name core $(rustc --print sysroot)/lib/rustlib/src/rust/library/core/src/lib.rs

echo '#![allow(internal_features)]
' | RUSTC_BOOTSTRAP=1 rustdoc --edition=2021 --target=aarch64-unknown-none-softfloat --sysroot=/dev/null -Zfixed-x18 --extern core=libcore.rmeta -
```

will fail with:

```text
error: mixing `-Zfixed-x18` will cause an ABI mismatch in crate `rust_out`
  |
  = help: the `-Zfixed-x18` flag modifies the ABI so Rust crates compiled with different values of this flag cannot be used together safely
  = note: unset `-Zfixed-x18` in this crate is incompatible with `-Zfixed-x18=` in dependency `core`
  = help: set `-Zfixed-x18=` in this crate or unset `-Zfixed-x18` in `core`
  = help: if you are sure this will not cause problems, you may use `-Cunsafe-allow-abi-mismatch=fixed-x18` to silence this error
```

Thus save the targets modifiers in `Options` to then pass it to the
session options, so that eventually the diff can be performed as expected
in `report_incompatible_target_modifiers()`.

Fixes: https://github.com/rust-lang/rust/issues/144521
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

0 files changed, 0 insertions, 0 deletions