This change drops the support for FreeBSD 9 or below and simplifies
platform-dependent code for the sake of maintenance.

Updates #7187.
Fixes #11412.
Updates #16064.
Updates #18854.
Fixes #19072.

Change-Id: I9129130aafbfc7d0d7e9b674b6fc6cb31b7381be
Reviewed-on: https://go-review.googlesource.com/64910Reviewed-by: Ian Lance Taylor <iant@golang.org>