test(build-std): make mock-std closer to real world

[weihanglo]

What does this PR try to resolve?

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.

How should we test and review this PR?

Change this line to .build_std_arg(&setup, "core").

cargo/tests/testsuite/standard_lib.rs

Line 410 in 05f54fd

.build_std(&setup)

And delete these liens:

cargo/src/cargo/core/compiler/standard_lib.rs

Lines 75 to 84 in 05f54fd

	// check that targets support building std
	if crates.contains(&"std".to_string()) {
	let unsupported_targets = target_data.get_unsupported_std_targets();
	if !unsupported_targets.is_empty() {
	anyhow::bail!(
	"building std is not supported on the following targets: {}",
	unsupported_targets.join(", ")
	)
	}
	}

Then the test project should compile.

[rustbot]

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[weihanglo]

We still need {aarch64,x86_64}-unknown-none targets installed to make test_std_on_unsupported_target compile. We need to tweak cargo CI and cargo-test-macro to support this use case. That is out of scope of this PR.