[lld] respect LLVM_EXTERNAL_LIT

Consider a setup without a system-wide installation of lit.
Instead you pass the path to lit like this:

```
cmake ...
-DLLVM_EXTERNAL_LIT=<PATH_TO_LIT_BINARY> ...
```

Then you will run into this error:

```
ninja: error: unknown target 'check-lld'
```

I have a buildbot builder that fails with this message. Here's the
passage that triggers this error:

https://github.com/llvm/llvm-zorg/blob/d3bfd5ccbceb542098c350e4d071ceceac6854cb/zorg/buildbot/builders/annotated/standalone-build.sh#L194-L239

By using `LLVM_EXTERNAL_LIT` instead of `LLVM_LIT` we fix this problem.

See
[here](https://llvm.org/docs/GettingStarted.html#stand-alone-builds) for
a description:

> Both the LLVM_ROOT and LLVM_EXTERNAL_LIT options are required to do stand-alone builds for all sub-projects. Additional required options for each sub-project can be found in the table below.

Differential Revision: https://reviews.llvm.org/D154599

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