unix: keep generated types compilable with current cgo

I hit this while regenerating ztypes for a platform that is not in the tree
yet, but it is not specific to that: any regeneration with a current
toolchain runs into it.

Where `struct fp_extended_precision` is not declared, `cgo -godefs` now
renders `FpExtendedPrecision` as `_cgopackage.Incomplete`, and the result
does not compile. The committed files for those platforms carry an empty
struct, so this keeps mkpost producing what is already there.

freebsd/arm is unaffected: the C type is declared there, so cgo never emits
`Incomplete` for it.

Change-Id: Ic77656dca440cad8a63ea9ac84c06b326d174cc4
GitHub-Last-Rev: 05487a4adc1bf25bbbab471c82ee0a58235384a1
GitHub-Pull-Request: golang/sys#285
Reviewed-on: https://go-review.googlesource.com/c/sys/+/816660
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>