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

[Brad Smith via cfe-commits]

brad0 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.

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