Add option to force filetype=asm for testing

Makefile.standalone

Index: Makefile.standalone
diff --git a/Makefile.standalone b/Makefile.standalone
index eb9a3aa61b7272c9ad01f4a40886afa4d9cfac47..9e423c3de256800abd0a82bb39308c23cf3a234f 100644
--- a/Makefile.standalone
+++ b/Makefile.standalone
@@ -127,6 +127,19 @@ ifdef MSAN
   LD_EXTRA += -fsanitize=memory
 endif
 
+ifdef FORCEASM
+  FORCEASM_FLAG = --filetype=asm
+  # With --filetype=asm and --sandbox, the llvm-mc assembler emits the lock and
+  # 16-bit prefixes in the "wrong" order, causing the validator to reject the
+  # resulting nexe.  So we just disable those tests for now.
+  FORCEASM_XTEST_EXCLUDES = -e x8632,sandbox,test_sync_atomic
+  FORCEASM_LIT_PARAM = --param=FORCEASM
+else
+  FORCEASM_FLAG =
+  FORCEASM_XTEST_EXCLUDES =
+  FORCEASM_LIT_PARAM =
+endif
+
 SB_OBJDIR := $(OBJDIR)+Sandboxed
 
 $(info -----------------------------------------------)
@@ -392,7 +405,7 @@ runtime.is.built: $(RT_SRC) pydir/build-runtime.py
 
 check-lit: $(OBJDIR)/pnacl-sz make_symlink
 	PNACL_BIN_PATH=$(PNACL_BIN_PATH) \
-	$(LLVM_SRC_PATH)/utils/lit/lit.py -sv tests_lit
+	$(LLVM_SRC_PATH)/utils/lit/lit.py -sv tests_lit $(FORCEASM_LIT_PARAM)
 
 ifdef MINIMAL
 check-xtest: $(OBJDIR)/pnacl-sz make_symlink runtime
@@ -405,6 +418,8 @@ check-xtest: $(OBJDIR)/pnacl-sz make_symlink runtime
        # TODO(jpp): reenable the x86-64 tests.
 	./pydir/crosstest_generator.py -v --lit \
           --toolchain-root $(TOOLCHAIN_ROOT) \
+          $(FORCEASM_FLAG) \
+          $(FORCEASM_XTEST_EXCLUDES) \
           -i x8632,native,sse2 \
           -i x8632,native,sse4.1,test_vector_ops \
           -i x8632,sandbox,sse4.1,Om1 \
@@ -470,7 +485,7 @@ check-presubmit presubmit:
 # Check the x86 assembler unit tests.
 	+make -f Makefile.standalone \
           DEBUG=1 CHECK_X86_ASM=1 check-unit
-# Run spec2k for x86-32.
+# Run lit tests, cross tests, unit tests, and spec2k/x86-32.
 	+make -f Makefile.standalone \
           check check-spec
 # Build spec2k under -Om1/x86-32, to check for liveness errors.
@@ -479,6 +494,9 @@ check-presubmit presubmit:
 # Run spec2k for x86-32 without advanced phi lowering.
 	+make -f Makefile.standalone \
           SPECFLAGS='--sz=--phi-edge-split=0' check-spec
+# Run cross tests and lit tests to validate filetype=asm output.
+	+make -f Makefile.standalone \
+          FORCEASM=1 check-xtest check-lit
 # Build spec2k for arm32.
 	+make -f Makefile.standalone \
           TARGET=arm32 SPECBUILDONLY=true check-spec