[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

Author: mgorny

libcxx/cmake/Modules/HandleLibCXXABI.cmake

@@ -118,6 +118,10 @@ elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libcxxabi")
   setup_abi_lib(
     "-DLIBCXX_BUILDING_LIBCXXABI"
     "${shared}" "${static}" "cxxabi.h;__cxxabi_config.h" "")
+elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "system-libcxxabi")
+  setup_abi_lib(
+    "-DLIBCXX_BUILDING_LIBCXXABI"
+    "c++abi" "c++abi" "cxxabi.h;__cxxabi_config.h" "")
 elseif ("${LIBCXX_CXX_ABI_LIBNAME}" STREQUAL "libcxxrt")
   if(NOT LIBCXX_CXX_ABI_INCLUDE_PATHS)
     set(LIBCXX_CXX_ABI_INCLUDE_PATHS "/usr/include/c++/v1")

libcxx/docs/BuildingLibcxx.rst

@@ -318,7 +318,7 @@ ABI Library Specific Options
 
 .. 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.
 

libcxx/utils/libcxx/test/config.py

@@ -406,6 +406,8 @@ def configure_link_flags_abi_library(self):
                         self.cxx.link_flags += [abs_path]
                     else:
                         self.cxx.link_flags += ['-lc++abi']
+        elif cxx_abi == 'system-libcxxabi':
+            self.cxx.link_flags += ['-lc++abi']
         elif cxx_abi == 'libcxxrt':
             self.cxx.link_flags += ['-lcxxrt']
         elif cxx_abi == 'vcruntime':