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>
diff --git a/unix/ztypes_freebsd_arm.go b/unix/ztypes_freebsd_arm.go
index e27511a..fc71399 100644
--- a/unix/ztypes_freebsd_arm.go
+++ b/unix/ztypes_freebsd_arm.go
@@ -64,45 +64,45 @@
 )
 
 type Stat_t struct {
-	Dev      uint64
-	Ino      uint64
-	Nlink    uint64
-	Mode     uint16
-	_0       int16
-	Uid      uint32
-	Gid      uint32
-	_1       int32
-	Rdev     uint64
-	Atim     Timespec
-	Mtim     Timespec
-	Ctim     Timespec
-	Birthtim Timespec
-	Size     int64
-	Blocks   int64
-	Blksize  int32
-	Flags    uint32
-	Gen      uint64
-	Spare    [10]uint64
+	Dev     uint64
+	Ino     uint64
+	Nlink   uint64
+	Mode    uint16
+	_0      int16
+	Uid     uint32
+	Gid     uint32
+	_1      int32
+	Rdev    uint64
+	Atim    Timespec
+	Mtim    Timespec
+	Ctim    Timespec
+	Btim    Timespec
+	Size    int64
+	Blocks  int64
+	Blksize int32
+	Flags   uint32
+	Gen     uint64
+	Spare   [10]uint64
 }
 
 type stat_freebsd11_t struct {
-	Dev      uint32
-	Ino      uint32
-	Mode     uint16
-	Nlink    uint16
-	Uid      uint32
-	Gid      uint32
-	Rdev     uint32
-	Atim     Timespec
-	Mtim     Timespec
-	Ctim     Timespec
-	Size     int64
-	Blocks   int64
-	Blksize  int32
-	Flags    uint32
-	Gen      uint32
-	Lspare   int32
-	Birthtim Timespec
+	Dev     uint32
+	Ino     uint32
+	Mode    uint16
+	Nlink   uint16
+	Uid     uint32
+	Gid     uint32
+	Rdev    uint32
+	Atim    Timespec
+	Mtim    Timespec
+	Ctim    Timespec
+	Size    int64
+	Blocks  int64
+	Blksize int32
+	Flags   uint32
+	Gen     uint32
+	Lspare  int32
+	Btim    Timespec
 }
 
 type Statfs_t struct {