Fix empty target_config in apply_rust_config bootstrap

[Shourya742]

This PR fixes the issue of an empty target_config in apply_rust_config, which was caused by the ordering of TOML config parsing. This was inadvertently introduced during the last config refactor. The test and the corresponding configuration order have been corrected in this PR.

r? @Kobzol

[rustbot]

This PR modifies src/bootstrap/src/core/config.

If appropriate, please update CONFIG_CHANGE_HISTORY in src/bootstrap/src/utils/change_tracker.rs.

[Kobzol]

Hmm, I don't like this. Let's figure out a different way to do this. Because the config hardcodes the name of a specific target, we'll have to generalize that somehow. For example, we can create a function that will return a list of targets for which we opt into LLD by default. Then in tests, we'll have a #[cfg(test)] version of that function, which will temporarily allow a test to override this set of targets.

Then we should implement explicit tests that will override the LLD opt in target list and check that bootstrap behaves correctly, i.e. disables the opt in if external LLVM config is provided and if LLD is disabled in the config.