r180783 - Always use / when computing mips specific paths.

[Rafael Espíndola]

>> We were getting paths with both / and \ in them. This should fix
>> mips-cs-ld.c
>> on the windows bots.
>
>
> You also removed a bunch of '.str()' method calls here, which is good but
> should be mentioned in the commit or committed in a separate patch.

It seemed incidental given the change to std::string&. I will try to
remember it in future changes.

Cheers,
Rafael