[SystemZ] Add header files to Makefile / module.modulemap

This should fix build-bot failures after r233804.

The patch also adds a "systemz" feature, and renames the
"transactional-execution" feature to "htm", since it turns
out "-" is not a legal character in module feature names.

llvm-svn: 233807
diff --git a/clang/lib/Basic/Targets.cpp b/clang/lib/Basic/Targets.cpp
index 48a3a7c..96dda54e 100644
--- a/clang/lib/Basic/Targets.cpp
+++ b/clang/lib/Basic/Targets.cpp
@@ -5487,7 +5487,8 @@
 
   bool hasFeature(StringRef Feature) const override {
     return llvm::StringSwitch<bool>(Feature)
-        .Case("transactional-execution", HasTransactionalExecution)
+        .Case("systemz", true)
+        .Case("htm", HasTransactionalExecution)
         .Default(false);
   }
 };