Switch libc++/libc++abi builders to the new check-meow targets

The check-libcxx and check-libcxxabi targets are deprecated in favour of
the check-cxx and check-cxxabi targets, respectively.
diff --git a/zorg/buildbot/builders/LibcxxAndAbiBuilder.py b/zorg/buildbot/builders/LibcxxAndAbiBuilder.py
index b018387..34ee8da 100644
--- a/zorg/buildbot/builders/LibcxxAndAbiBuilder.py
+++ b/zorg/buildbot/builders/LibcxxAndAbiBuilder.py
@@ -102,7 +102,7 @@
     # Test libc++abi
     f.addStep(LitTestCommand(
         name            = 'test.libcxxabi',
-        command         = ['make', jobs_flag, 'check-libcxxabi'],
+        command         = ['make', jobs_flag, 'check-cxxabi'],
         description     = ['testing', 'libcxxabi'],
         descriptionDone = ['test', 'libcxxabi'],
         workdir         = build_path))
@@ -110,7 +110,7 @@
     # Test libc++
     f.addStep(LitTestCommand(
         name            = 'test.libcxx',
-        command         = ['make', jobs_flag, 'check-libcxx'],
+        command         = ['make', jobs_flag, 'check-cxx'],
         description     = ['testing', 'libcxx'],
         descriptionDone = ['test', 'libcxx'],
         workdir         = build_path))