Rename disassembly file to fix testing with IAR

Disassembly files can unintentionally get picked up by the test build
system.  This patch changes the file extension on those files from
".s" to ".s.txt" so they are ignored.

tools/toolchains/iar.py

@@ -104,7 +104,7 @@ class IAR(mbedToolchain):
 
     def cc_extra(self, object):
         base, _ = splitext(object)
-        return ["-l", base + '.s']
+        return ["-l", base + '.s.txt']
 
     def get_compile_options(self, defines, includes):
         return ['-D%s' % d for d in defines] + ['-f', self.get_inc_file(includes)]