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/configure b/configure
index 496854d..7d16620 100755
--- a/configure
+++ b/configure
@@ -2337,6 +2337,8 @@
     llvm_cv_target_os_type="MingW" ;;
   *-*-haiku*)
     llvm_cv_target_os_type="Haiku" ;;
+  *-*-rtems*)
+    llvm_cv_target_os_type="RTEMS" ;;
   *-unknown-eabi*)
     llvm_cv_target_os_type="Freestanding" ;;
   *)