Add top-level CMake 'compiler-rt' target to build all compiler-rt libraries

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@191747 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4a7be9c..123b60e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -15,6 +15,9 @@
 # runtime libraries.
 cmake_minimum_required(VERSION 2.8.8)
 
+# Top level target used to build all compiler-rt libraries.
+add_custom_target(compiler-rt)
+
 # Compute the Clang version from the LLVM version.
 # FIXME: We should be able to reuse CLANG_VERSION variable calculated
 #        in Clang cmake files, instead of copying the rules here.