unix: rename Stat_t time fields to [AMCB]tim

Birthtime/Birthtimespec was renamed to Btim to match the others.
Also unexport the [ACMB]tim_ext fields on freebsd 386.
Update mkpost.go to do the renames.

Ran the following on the existing ztypes_*.go files:

  #!/bin/sh
  set -e

  for f in ztypes_*.go; do
      cat $f | go run mkpost.go | gofmt > $f.tmp
      mv $f.tmp $f
  done;

  cat ztypes_freebsd_386.go | env GOOS=freebsd GOARCH=386 go run mkpost.go | gofmt > ztypes_freebsd_386.go.tmp
  mv ztypes_freebsd_386.go{.tmp,}

Fixes golang/go#31735

Change-Id: I15765d690ee8d2be6bbb37f465322bc019722e08
Reviewed-on: https://go-review.googlesource.com/c/sys/+/175157
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/syscall_freebsd.go

@@ -404,22 +404,22 @@ func roundup(x, y int) int {
 
 func (s *Stat_t) convertFrom(old *stat_freebsd11_t) {
 	*s = Stat_t{
-		Dev:      uint64(old.Dev),
-		Ino:      uint64(old.Ino),
-		Nlink:    uint64(old.Nlink),
-		Mode:     old.Mode,
-		Uid:      old.Uid,
-		Gid:      old.Gid,
-		Rdev:     uint64(old.Rdev),
-		Atim:     old.Atim,
-		Mtim:     old.Mtim,
-		Ctim:     old.Ctim,
-		Birthtim: old.Birthtim,
-		Size:     old.Size,
-		Blocks:   old.Blocks,
-		Blksize:  old.Blksize,
-		Flags:    old.Flags,
-		Gen:      uint64(old.Gen),
+		Dev:     uint64(old.Dev),
+		Ino:     uint64(old.Ino),
+		Nlink:   uint64(old.Nlink),
+		Mode:    old.Mode,
+		Uid:     old.Uid,
+		Gid:     old.Gid,
+		Rdev:    uint64(old.Rdev),
+		Atim:    old.Atim,
+		Mtim:    old.Mtim,
+		Ctim:    old.Ctim,
+		Btim:    old.Btim,
+		Size:    old.Size,
+		Blocks:  old.Blocks,
+		Blksize: old.Blksize,
+		Flags:   old.Flags,
+		Gen:     uint64(old.Gen),
 	}
 }