rust: Suppress searching builtin sysroot

By default, if Rust is passed `--target=foo` rather than a target.json
file, it will infer a default sysroot if that component is installed. As
the proposed aarch64 support [1] uses `aarch64-unknown-none` rather than a
target.json file, this is needed [2] to prevent rustc from being confused
between the custom kernel sysroot and the pre-installed one.

[ Miguel: Applied Boqun's extra case (for `rusttest`) and reworded to add
  links to the arm64 patch series discussion. In addition, fixed the
  `rustdoc` target too (which requires a conditional since `cmd_rustdoc`
  is also used for host crates like `macros`). ]

Signed-off-by: Matthew Maurer <[email protected]>
Tested-by: Boqun Feng <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Link: https://lore.kernel.org/rust-for-linux/[email protected]/ [1]
Link: https://lore.kernel.org/rust-for-linux/CAGSQo01pOixiPXkW867h4vPUaAjtKtHGKhkV-rpifJvKxAf4Ww@mail.gmail.com/ [2]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Miguel Ojeda <[email protected]>

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