PR #38842 has exposed that we were missing the src/test/compile-fail-fulldeps
directory in the search for feature gate tests. Because the detection didn't
work despite the effort to name the test appropriately and add a correct
"// gate-test-proc_macro" comment, proc_macro was added to the whitelist.

We fix this little weakness in the feature gate tidy check and add
the src/test/compile-fail-fulldeps directory to the checked directories.

This fixes the final issues with the target related to unwinding by disabling
removal of frame pointers.

Removal of the lang feature gate tests whitelist #39059

r? @est31

We're not using it in the examples anymore.

See #38644.

This reverts commit fe9f5d52.

They're long dead since the switch to flexible targets, but was not
removed like their consumers were. Interesting they even got maintained
by various porters out there!

Technically [syntax-breaking] as they're public API, but since they're
unused in the compiler, the potential breakage IMO should be minimal.

The diagnostic emitter now accounts for labels with no text message,
presenting the underline on its own, without drawing the line for the
non existing message below it. Go from

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ----^^^^^^^----
  |   |   |
  |   |   `b` is a good letter
  |
```

to

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ----^^^^^^^----
  |       |
  |       `b` is a good letter
```

and from

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ^^^^-------^^^^
  |   |   |
  |   |
  |   `a` is a good letter
```

to

```
error: foo
 --> test.rs:3:6
  |
3 |   a { b { c } d }
  |   ^^^^-------^^^^ `a` is a good letter
```

Similar to the makefiles, pass CFG_LIBDIR_RELATIVE to cargo when building
rustc in stages > 0.  This tells rustc to check the different directory.

Also fix a typo which linkchecker should have caught.