tidy up a comment (#315)

emsdk

@@ -82,7 +82,8 @@ if not OSX and (platform.system() == 'Linux' or os.name == 'posix'):
 UNIX = (OSX or LINUX)
 
 ARCH = 'unknown'
-machine = platform.machine().lower() #amd64 was not being recognised when returned as AMD64
+# platform.machine() may return AMD64 on windows, so standardize the case.
+machine = platform.machine().lower()
 if machine.startswith('x64') or machine.startswith('amd64') or machine.startswith('x86_64'):
   ARCH = 'x86_64'
 elif machine.endswith('86'):