[SubZero] Generate MIPS.abiflags section

The patch generates MIPS.abiflags section. This section contains a
versioned data structure with essential information required for
loader to determine the requirements of the application.

R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/2471883005 .

Patch from Jaydeep Patil <jaydeep.patil@imgtec.com>.
diff --git a/src/IceTargetLowering.h b/src/IceTargetLowering.h
index 568b81f..3de6965 100644
--- a/src/IceTargetLowering.h
+++ b/src/IceTargetLowering.h
@@ -616,6 +616,7 @@
                             const std::string &SectionSuffix) = 0;
   virtual void lowerConstants() = 0;
   virtual void lowerJumpTables() = 0;
+  virtual void emitTargetRODataSections() {}
 
 protected:
   void emitGlobal(const VariableDeclaration &Var,