[clang] [Driver] Link Flang runtime on Solaris (PR #65644)

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> > Agreed: AFAICS the only open issue is whether the Solaris test should use the `GNU` label as I have done, introduce an new common one (like `UNIX`; there's nothing GNU-specific in that test), or really introduce a separate copy of the check under a different label per target (my least preference because it makes the test hard to read for no gain).
> 
> I think having additional tests makes sense if there is some variation on what is being checked but not when it's just copying and pasting the same thing with a different label.

I feel that we are bike-shedding here a bit. Please prioritise correctness - using `GNU` as a label for Solaris would not be correct. The following would be:
* duplicating tests with a different label (e.g. `SOLARIS`) would be correct,
* renaming the current label from `GNU` to e.g. `UNIX` would be correct.

Unless I am missing something?

https://github.com/llvm/llvm-project/pull/65644