After CL 136816 the bits member of the FdSet struct is no longer
exported. Make sure to retain it in mkpost.go and rename it to Bits
as on all other systems.

Change-Id: I860b307a80c7e29b4a75d9d916d879ee0a31c014
Reviewed-on: https://go-review.googlesource.com/c/149897
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Yuval Pavel Zholkover <paulzhol@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>