[PATCH] Add option to use temporary file for assembling with clang

[David Peixotto]

> It seems a reasonable dev option to have.
> * Can you test that the file is deleted?

Is there an easy way to test for this? I looked for another example and could not find one. I can think of writing a test that executes a compilation with that flag and -v, then looks to see if the output file exists. I see a couple of issues with that test, however

1. It will only work for a specific target because it requires compilation to complete (e.g. REQUIRES: arm-registered-target)

2. How do I make a cross-platform check to see if a file exists? I could use ls or or something on linux/darwin, but what about windows?

Is it ok for the test to work for a specific target and exclude windows (is there a way to do that?)? Or is there a better way to write the test?

> * Do we have a more regular naming patter for developer options?
> Should this be a -ccc-* option? Should be be a InternalDebugOpt?

I welcome suggestions on naming the flag and putting it in the right group.