implement the null function for the Mips32 subzero compiler

Makefile.standalone

 # The following variables will likely need to be modified, depending on where
 # and how you built LLVM & Clang. They can be overridden in a command-line
 # invocation of make, like:
 #
 #   make LLVM_SRC_PATH=<path> LIBCXX_INSTALL_PATH=<path> CLANG_PATH=<path> \
 #        PNACL_BIN_PATH=<path> ...
 #
 
 # LLVM_SRC_PATH is the path to the root of the checked out source code. This
 # directory should contain the configure script, the include/ and lib/
@@ skipping 171 matching lines @@
         IceClFlags.cpp \
         IceCompiler.cpp \
         IceCompileServer.cpp \
         IceELFObjectWriter.cpp \
         IceELFSection.cpp \
         IceFixups.cpp \
         IceGlobalContext.cpp \
         IceGlobalInits.cpp \
         IceInst.cpp \
         IceInstARM32.cpp \
+        IceInstMIPS32.cpp \
         IceInstX8632.cpp \
         IceIntrinsics.cpp \
         IceLiveness.cpp \
         IceOperand.cpp \
         IceRegAlloc.cpp \
         IceRNG.cpp \
         IceTargetLowering.cpp \
         IceTargetLoweringARM32.cpp \
         IceTargetLoweringMIPS32.cpp \
         IceTargetLoweringX8632.cpp \
@@ skipping 155 matching lines @@
 
 docs:
         doxygen Doxyfile
         @echo See file://`pwd`/docs/html/index.html
 
 clean:
         rm -rf pnacl-sz *.o $(OBJDIR) $(SB_OBJDIR) build/pnacl-sz.bloat.json
 
 clean-all: clean
         rm -rf build/ docs/