libcxx: Fix ARM libc++/abi and libunwind buildbot.

[logan]

Summary

The test cases were crashing due to the mixed usage of the unwinding
functions from both libunwind and libgcc_s. The unwind functions are
mixed because the "llvm_unwinder" entry is not available in the
lit.site.cfg for libc++. As a result, "-lgcc_s" is picked instead of
"-lunwind". The extra option to lit --param=link_flags="-lunwind" won't
help either.

This CL fix the problem by adding llvm_unwinder to lit.site.cfg.in.

[rengolin]

YAY! If that fixes the bots, and continue testing what it was supposed to test (libcxxabi+libunwind), I'm happy. :)

[logan]

Thanks for your review. Committed as rL237518.

Let's see if it works.

CMakeLists.txt

if (NOT LIBCXX_CXX_ABI)
   endif ()
 else ()
   set(LIBCXX_CXX_ABI_LIBNAME "${LIBCXX_CXX_ABI}")
 endif ()
 set(LIBCXX_CXX_ABI "${LIBCXX_CXX_ABI}" CACHE STRING
     "Specify C++ ABI library to use." FORCE)
 set_property(CACHE LIBCXX_CXX_ABI PROPERTY STRINGS ;${CXXABIS})
 
+# Build libc++abi with libunwind. We need this option to determine whether to
+# link with libunwind or libgcc_s while running the test cases.
+option(LIBCXXABI_USE_LLVM_UNWINDER "Build and use the LLVM unwinder." OFF)
+
 # TODO(ericwf): Remove these options by March/25/2015.
 # These CMake options have been removed in favor of a generic option
 # "LIBCXX_CXX_ABI_INCLUDE_PATHS". Issue an explicit error message explaining
 # the change for anybody that still uses them.
 if (DEFINED LIBCXX_LIBCXXABI_INCLUDE_PATHS)
   message(FATAL_ERROR
     "The option \"LIBCXX_LIBCXXABI_INCLUDE_PATHS\" has been removed. "
     "Use \"LIBCXX_CXX_ABI_INCLUDE_PATHS\" instead "

test/lit.site.cfg.in

 config.abi_library_path         = "@LIBCXX_CXX_ABI_LIBRARY_PATH@"
 config.configuration_variant    = "@LIBCXX_LIT_VARIANT@"
 config.target_triple            = "@LIBCXX_TARGET_TRIPLE@"
 config.sysroot                  = "@LIBCXX_SYSROOT@"
 config.gcc_toolchain            = "@LIBCXX_GCC_TOOLCHAIN@"
 config.generate_coverage        = "@LIBCXX_GENERATE_COVERAGE@"
 config.target_info              = "@LIBCXX_TARGET_INFO@"
 config.executor                 = "@LIBCXX_EXECUTOR@"
+config.llvm_unwinder            = "@LIBCXXABI_USE_LLVM_UNWINDER@"
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")