Initialize LLVM's AMDGPU target machine, if available.

Note this isn't useful, yet. More changes will be necessary to be able to
actually codegen for this machine. As such, it is not enabled by default.

This patch is on its own for the benefit of the reviewers.

src/librustc_llvm/lib.rs

@@ -331,6 +331,12 @@ pub fn initialize_available_targets() {
                  LLVMInitializeAArch64TargetMC,
                  LLVMInitializeAArch64AsmPrinter,
                  LLVMInitializeAArch64AsmParser);
+    init_target!(llvm_component = "amdgpu",
+                 LLVMInitializeAMDGPUTargetInfo,
+                 LLVMInitializeAMDGPUTarget,
+                 LLVMInitializeAMDGPUTargetMC,
+                 LLVMInitializeAMDGPUAsmPrinter,
+                 LLVMInitializeAMDGPUAsmParser);
     init_target!(llvm_component = "mips",
                  LLVMInitializeMipsTargetInfo,
                  LLVMInitializeMipsTarget,