[libc++] Revisit libc++'s ABI list mechanism

Our current ABI list mechanism is flawed: it hardcodes some symbols to ignore/not-ignore from libc++abi. It's also pretty complicated due to trying to be platform agnostic (which works so-so in the end). Instead, we could potentially simplify these tests by making them more platform specific and writing them in Lit directly. Suggestion:

```
// REQUIRES: target={{.+}}-linux-?? && libcpp-abi-whatever

RUN: nm -omg %{libcxx-dylib} | canonicalization > %t.actual.txt
RUN: split-file
RUN: diff %t.actual.txt %t.expected.txt

//--- %t.expected.txt ---
HERE LIST OF SYMBOLS / OUTPUT OF NM
////
```