Adapt the -mfix-loongson2f-btb workaround from as(1) to LLVM/clang.

Index: lib/Target/Mips/Mips.h
--- lib/Target/Mips/Mips.h.orig
+++ lib/Target/Mips/Mips.h
@@ -38,6 +38,7 @@ namespace llvm {
   FunctionPass *createMicroMipsSizeReducePass();
   FunctionPass *createMipsExpandPseudoPass();
   FunctionPass *createMipsPreLegalizeCombiner();
+  FunctionPass *createMipsLoongson2FBTBFix();
 
   InstructionSelector *createMipsInstructionSelector(const MipsTargetMachine &,
                                                      MipsSubtarget &,
@@ -47,6 +48,7 @@ namespace llvm {
   void initializeMipsBranchExpansionPass(PassRegistry &);
   void initializeMicroMipsSizeReducePass(PassRegistry &);
   void initializeMipsPreLegalizerCombinerPass(PassRegistry&);
+  void initializeMipsLoongson2FBTBFixPass(PassRegistry &);
 } // end namespace llvm;
 
 #endif