PTX support

- `--emit=asm --target=nvptx64-nvidia-cuda` can be used to turn a crate
  into a PTX module (a `.s` file).

- intrinsics like `__syncthreads` and `blockIdx.x` are exposed as
  `"platform-intrinsics"`.

- "cabi" has been implemented for the nvptx and nvptx64 architectures.
  i.e. `extern "C"` works.

- a new ABI, `"ptx-kernel"`. That can be used to generate "global"
  functions. Example: `extern "ptx-kernel" fn kernel() { .. }`. All
  other functions are "device" functions.

src/librustc_llvm/lib.rs

@@ -377,6 +377,11 @@ pub fn initialize_available_targets() {
                  LLVMInitializeSparcTargetMC,
                  LLVMInitializeSparcAsmPrinter,
                  LLVMInitializeSparcAsmParser);
+    init_target!(llvm_component = "nvptx",
+                 LLVMInitializeNVPTXTargetInfo,
+                 LLVMInitializeNVPTXTarget,
+                 LLVMInitializeNVPTXTargetMC,
+                 LLVMInitializeNVPTXAsmPrinter);
 }
 
 pub fn last_error() -> Option<String> {