[patch] add function for system argument limit checking

[Rafael Espíndola]

>> Since this is for use in clang, lets test it there.
>
> OK.  I haven't worked out how to test this in a platform-independent
> manner; if you have ideas, I'd be interested in hearing them.

A somewhat fuzzy but still useful test is to check that the driver
works when given a command line that is too long on some on some
architectures. When running on those architectures the test will check
the @file creation. When running on an architecture with a bigger
limit the test will be a nop.

>> Do you want me to commit this? Depending on how much work you expect
>> to do you might want to ask for commit access.
>
> Please commit it for me.  I'll ask for commit access if/when having
> other people commit patches becomes a burden.

Done.

> Thanks,
> -Nathan

Thanks,
Rafael