Propagate InstructionSetFeatures to ElfBuilder.

This is subset of CL171665 and it separates it into two.
It will be needed to generate .MIPS.abiflags ELF section.

Change-Id: I5557e7cb98d0fa1dc57c85cf6161e119c6d50a1a
diff --git a/compiler/jit/jit_compiler.h b/compiler/jit/jit_compiler.h
index bc134fe..533dccf 100644
--- a/compiler/jit/jit_compiler.h
+++ b/compiler/jit/jit_compiler.h
@@ -42,6 +42,9 @@
   CompilerOptions* GetCompilerOptions() const {
     return compiler_options_.get();
   }
+  CompilerDriver* GetCompilerDriver() const {
+    return compiler_driver_.get();
+  }
 
  private:
   std::unique_ptr<CompilerOptions> compiler_options_;