Remove legacy CMake targets for libcxx and libcxxabi

We've been meaning to remove those targets for a while, and the fix is
simple enough cause they're all just aliases to other targets.

There's no doubt this commit will break some CI systems, however the
fix is trivial.

Differential Revision: https://reviews.llvm.org/D76104

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f383fb40b17b6b39b37da588ff3c8a2972c685ad
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index f2fff0e..ab5c72d 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -68,7 +68,7 @@
           -DLIBCXX_CXX_ABI_INCLUDE_PATHS=path/to/separate/libcxxabi/include \
           path/to/llvm-project/libcxx
   $ make
-  $ make check-libcxx # optional
+  $ make check-cxx # optional
 
 
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