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

I noticed that `flang-new` cannot link Fortran executables on Solaris since the runtime libs are missing.

This patch fixes this, following `Gnu.cpp`. The `linker-flags.f90` testcase is augmented to test for this,
renaming the `GNU` label to `UNIX` so it can be reused.  Also use the current form `--target=` in the tests
and join the `-l` lines in the test for readibility.

Tested on `amd64-pc-solaris2.11`, `sparcv9-sun-solaris2.11`, and
`x86_64-pc-linux-gnu`.