Make R600 non-experimental.

The r600 backend has been in tree for some time now. Marking it as
non-experimental to avoid accidental breakage.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182442 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 2352fba..18a172a 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -83,6 +83,7 @@
   MSP430
   NVPTX
   PowerPC
+  R600
   Sparc
   SystemZ
   X86