[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:

d3bfd5ccbc/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

lld/CMakeLists.txt

@@ -61,7 +61,7 @@ if(LLD_BUILT_STANDALONE)
 
     if(EXISTS ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
       # Note: path not really used, except for checking if lit was found
-      set(LLVM_LIT ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
+      set(LLVM_EXTERNAL_LIT ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
       if(NOT LLVM_UTILS_PROVIDED)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/FileCheck utils/FileCheck)
         add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/not utils/not)
@@ -74,13 +74,13 @@ if(LLD_BUILT_STANDALONE)
       endif()
     else()
       # Seek installed Lit.
-      find_program(LLVM_LIT
+      find_program(LLVM_EXTERNAL_LIT
                    NAMES llvm-lit lit.py lit
                    PATHS "${LLVM_MAIN_SRC_DIR}/utils/lit"
                    DOC "Path to lit.py")
     endif()
 
-    if(LLVM_LIT)
+    if(LLVM_EXTERNAL_LIT)
       # Define the default arguments to use with 'lit', and an option for the user
       # to override.
       set(LIT_ARGS_DEFAULT "-sv")