`test_std_on_unsupported_target` never really succeed to build those targets, due to
* local rustup may not have `{aarch64,x86_64}-unknown-none` installed.
* `core` and `compiler-builtins` mock crate are not `no_std` nor `no_core`
* the dummy `main.rs` uses `println!` and is not `no_std`.
This commit make it compile, if you have those targets installed.
Add a test case which ensures that -Zbuild-std without --target
correctly handles building a crate that has a shared dependency between
it's own build script, and std.
Starting from rust-lang/rust#128534 (nightly-2024-08-05),
stnadard library has its own Cargo workspace.
Hence `-Zbuild-std` no longer need to fake a virtual workspace.
This also adjusts Cargo.toml in `mock-std` to align with std's Cargo.toml
