[llvm] [runtimes][cmake] Keep default PIC behavior consistent in standalone build (PR #205797)

[Louis Dionne via llvm-commits]

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@@ -205,6 +205,12 @@ endif()
 # Avoid checking whether the compiler is working.
 set(LLVM_COMPILER_CHECKED ON)
 
+# Match default PIC behavior between bootstrap and standalone runtime builds.
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ldionne wrote:

```suggestion
# Match default PIC behavior between bootstrap and runtime builds.
```

"Standalone" builds refer to something else, which is not supported anymore. Historically, that meant building runtimes by rooting the CMake invocation at e.g. `libcxx/CMakeLists.txt`. To avoid confusion, let's drop that terminology.

https://github.com/llvm/llvm-project/pull/205797