Subzero: Allow Makefile.standalone to build for a single target.

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 82 matching lines @@
     -DALLOW_LLVM_IR_AS_INPUT=1 -DALLOW_TIMERS=1 -DALLOW_MINIMAL_BUILD=0
 endif
 
 ifdef NODUMP
   OBJDIR := $(OBJDIR)+NoDump
   BASE_CXX_DEFINES += -DALLOW_DUMP=0
 else
   BASE_CXX_DEFINES += -DALLOW_DUMP=1
 endif
 
+# Restrict to a single supported target.  Current options:
+#   SZTARGET=ARM32
+#   SZTARGET=MIPS32
+#   SZTARGET=X8632
+#   SZTARGET=X8664
+ifdef SZTARGET
+  OBJDIR := $(OBJDIR)+T_$(SZTARGET)
+  BASE_CXX_DEFINES += -DSZTARGET=$(SZTARGET)
+endif
+
 CXX_DEFINES := $(BASE_CXX_DEFINES) -DPNACL_BROWSER_TRANSLATOR=0
 
 ifdef NOASSERT
   ASSERTIONS = -DNDEBUG
 else
   ASSERTIONS =
   OBJDIR := $(OBJDIR)+Asserts
 endif
 
 ifdef UBSAN
@@ skipping 549 matching lines @@
 
 help-check-xtest:
         @cat Makefile.standalone-help/check-xtest.txt
 
 clean:
         rm -rf pnacl-sz *.o $(foreach nexe,$(NEXES),$(notdir $(nexe))) \
           $(OBJDIR) $(SB_OBJDIR) $(SBB_OBJDIR) build/*.bloat.json
 
 clean-all: clean
         rm -rf build/ crosstest/Output/