[libcxx] Add an explicit option to build against system-libcxxabi

Add an explicit LIBCXX_CXX_ABI=system-libcxxabi option for linking to
system-installed libc++abi. This fixes the ability to link against one
when building libcxx via the runtimes build, as otherwise the build
system insists on linking into in-tree targets.

Differential Revision: https://reviews.llvm.org/D119539

NOKEYCHECK=True
GitOrigin-RevId: ba4f1e44e480d661d99973007a39dc642f5d79a4
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index aa26a22..0dd278e 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -318,7 +318,7 @@
 
 .. option:: LIBCXX_CXX_ABI:STRING
 
-  **Values**: ``none``, ``libcxxabi``, ``libcxxrt``, ``libstdc++``, ``libsupc++``.
+  **Values**: ``none``, ``libcxxabi``, ``system-libcxxabi``, ``libcxxrt``, ``libstdc++``, ``libsupc++``.
 
   Select the ABI library to build libc++ against.