Now that Fchmodat is available on all Unices, move TestFchmodat to
syscall_unix_test.go and adjust it such that Fchmodat with
AT_SYMLINK_NOFOLLOW in flags is tested where supported.

Change-Id: I41ef5b874e36d84ed4866706dbba075c39aa00d4
Reviewed-on: https://go-review.googlesource.com/101615
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>