initial support for s390x

A new target, `s390x-unknown-linux-gnu`, has been added to the compiler
and can be used to build no_core/no_std Rust programs.

Known limitations:

- librustc_trans/cabi_s390x.rs is missing. This means no support for
  `extern "C" fn`.
- No support for this arch in libc. This means std can be cross compiled
  for this target.

src/librustc_llvm/build.rs

@@ -66,7 +66,7 @@ fn main() {
     let host = env::var("HOST").unwrap();
     let is_crossed = target != host;
 
-    let optional_components = ["x86", "arm", "aarch64", "mips", "powerpc", "pnacl"];
+    let optional_components = ["x86", "arm", "aarch64", "mips", "powerpc", "pnacl", "systemz"];
 
     // FIXME: surely we don't need all these components, right? Stuff like mcjit
     //        or interpreter the compiler itself never uses.

src/librustc_llvm/lib.rs

@@ -428,6 +428,12 @@ pub fn initialize_available_targets() {
                  LLVMInitializePNaClTargetInfo,
                  LLVMInitializePNaClTarget,
                  LLVMInitializePNaClTargetMC);
+    init_target!(llvm_component = "systemz",
+                 LLVMInitializeSystemZTargetInfo,
+                 LLVMInitializeSystemZTarget,
+                 LLVMInitializeSystemZTargetMC,
+                 LLVMInitializeSystemZAsmPrinter,
+                 LLVMInitializeSystemZAsmParser);
 }
 
 pub fn last_error() -> Option<String> {