A couple of fixes for using Makefile.standalone on Mac.

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> LLVM_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/
 # directories of LLVM, Clang in tools/clang/, etc.
 # Alternatively, if you're building vs. a binary download of LLVM, then
 # LLVM_SRC_PATH can point to the main untarred directory.
 LLVM_SRC_PATH ?= ../llvm
 
 # LLVM_BIN_PATH is the directory where binaries are placed by the LLVM build
 # process. It should contain the tools like opt, llc and clang. The default
 # reflects a debug build with autotools (configure & make).
 LLVM_BIN_PATH ?= $(shell readlink -e \
         ../../out/llvm_i686_linux_work/Release+Asserts/bin)
 
+HOST_ARCH ?= x86
+ifeq ($(HOST_ARCH),x86_64)
+  HOST_FLAGS = -m64
+else
+  ifeq ($(HOST_ARCH),x86)
+    HOST_FLAGS = -m32
+  endif
+endif
+
 $(info -----------------------------------------------)
 $(info Using LLVM_SRC_PATH = $(LLVM_SRC_PATH))
 $(info Using LLVM_BIN_PATH = $(LLVM_BIN_PATH))
+$(info Using HOST_ARCH     = $(HOST_ARCH))
 $(info -----------------------------------------------)
 
 LLVM_CXXFLAGS := `$(LLVM_BIN_PATH)/llvm-config --cxxflags`
 LLVM_LDFLAGS := `$(LLVM_BIN_PATH)/llvm-config --ldflags --libs`
 
 # It's recommended that CXX matches the compiler you used to build LLVM itself.
 OPTLEVEL := -O0
 CXX := g++
+
 CXXFLAGS := -Wall -Wextra -Werror -fno-rtti -fno-exceptions \
-        $(OPTLEVEL) -g $(LLVM_CXXFLAGS) -m32 -Wno-error=unused-parameter
-LDFLAGS := -m32
+        $(OPTLEVEL) -g $(LLVM_CXXFLAGS) $(HOST_FLAGS) \
+        -Wno-error=unused-parameter
+LDFLAGS := $(HOST_FLAGS)
 
 SRCS= \
         IceCfg.cpp \
         IceCfgNode.cpp \
         IceConverter.cpp \
         IceGlobalContext.cpp \
         IceInst.cpp \
         IceInstX8632.cpp \
         IceIntrinsics.cpp \
         IceLiveness.cpp \
@@ skipping 44 matching lines @@
   CLANG_FORMAT_DIFF = clang-format-diff.py
 else
   CLANG_FORMAT_DIFF = /usr/lib/clang-format/clang-format-diff.py
 endif
 format-diff:
         git diff -U0 HEAD^ | \
         $(CLANG_FORMAT_DIFF) -p1 -style=LLVM -i
 
 clean:
         rm -rf llvm2ice *.o build/