[gfortran] gfortran.dg/dev_null.f90

[FX]

Testcase gfortran.dg/dev_null.f90 relies on a semantic for /dev/null
that is not correct for all Unices: it fails on all non-POSIX
complying /dev/null (all BSD, including darwin, and hpux).
However, on the OS where a POSIX behavior is expected (linux and
solaris, from what I've seen so far), this is a useful test.

I propose to add a target restriction to this test, so that it is run
only on *-*-linux and *-*-solaris*. Are you OK with that (for both
mainline and 4.0)? Any idea about others targets to add to the list?

FX