Explicitly filter any C-only cgo functions out of pclntable,
which allows them to be duplicated with the host binary.

Updates #18190.

Change-Id: I50d8706777a6133b3e95f696bc0bc586b84faa9e
Reviewed-on: https://go-review.googlesource.com/34199


Reviewed-by: Ian Lance Taylor <iant@golang.org>