On Illumos, TestFchmodat fails with:

--- FAIL: TestFchmodat (0.00s)
        syscall_unix_test.go:502: Fchmodat: unexpected error: operation not supported on transport endpoint

Like Linux, Illumos doesn't support flags != 0 in Fchmodat, see
https://illumos.org/man/2/chmod

Adjust TestFchmodat accordingly to handle EOPNOTSUPP on Illumos.

Change-Id: Icd4564497a41c4aa962cd76604b5ca2c7575d96c
Reviewed-on: https://go-review.googlesource.com/101775
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>