Add option to force filetype=asm for testing

tests_lit/lit.cfg

Index: tests_lit/lit.cfg
diff --git a/tests_lit/lit.cfg b/tests_lit/lit.cfg
index 3090e6063a70824a45a6da59ffbf821c97af0d8a..b49ee847009480061b0966ebce129a4138e738ff 100644
--- a/tests_lit/lit.cfg
+++ b/tests_lit/lit.cfg
@@ -89,6 +89,8 @@ iflc2i_atts_cmd = if_atts + [if_cond_flag('allow_llvm_ir_as_input'
 pnacl_sz_cmd = [os.path.join(pydir, 'run-pnacl-sz.py'),
                 '--pnacl-sz', pnacl_sz_tool,
                 '--pnacl-bin-path', pnaclbinpath]
+if 'FORCEASM' in lit_config.params:
+  pnacl_sz_cmd += ['--forceasm']
 
 # Run commands only if corresponding build attributes apply, including
 # for each compiler setup.