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

[Rainer Orth via cfe-commits]

rorth wrote:

> I'd like for this to move forward.

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).

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