Add initial *-*-rtems* target, from Joel Sherrill

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@134282 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/Triple.cpp b/lib/Support/Triple.cpp
index dbdb303..bf1fa09 100644
--- a/lib/Support/Triple.cpp
+++ b/lib/Support/Triple.cpp
@@ -113,6 +113,7 @@
   case Win32: return "win32";
   case Haiku: return "haiku";
   case Minix: return "minix";
+  case RTEMS: return "rtems";
   }
 
   return "<invalid>";