Rollup merge of #154993 - DeepeshWR:compiletest-custom-target-unstable, r=davidtwco

compiletest: pass -Zunstable-options for unpretty and no-codegen paths

When using custom targets via `RUST_TARGET_PATH`, compiletest does not
consistently pass `-Zunstable-options` in all code paths.

In particular, `unpretty` and `-Zno-codegen` (typecheck) paths invoke
rustc without `-Zunstable-options`, which causes failures when the
target is not built-in.

Pass `-Zunstable-options` in these code paths to ensure consistent
behavior when using custom targets.

This primarily affects setups using custom targets without `.json`
suffix (resolved via `RUST_TARGET_PATH`).

Author: JonathanBrouwer