Link `std` statically in `rustc_driver`
This makes `rustc_driver` statically link to `std`. This is done by not passing `-C prefer-dynamic` when building `rustc_driver`. However building `rustc-main` won't work currently as it tries to dynamically link to both `rustc_driver` and `std` resulting in a crate graph with `std` duplicated. To fix that new command line option `-Z prefer_deps_of_dynamic` is added which prevents linking to a dylib if there's a static variant of it already statically linked into another dylib dependency.
The main motivation for this change is to enable `#[global_allocator]` to be used in `rustc_driver` allowing overriding the allocator used in rustc on all platforms.
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Instead of adding `-Z prefer_deps_of_dynamic`, this PR is changed to crate opt-in to the linking change via the `rustc_private` feature instead, as that would be typically needed to link to `rustc_driver` anyway.
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try-job: aarch64-apple
try-job: x86_64-msvc
try-job: i686-mingw
try-job: dist-x86_64-msvc
try-job: aarch64-gnu
9cb1998ea1