MFC r291180:

MFC r291180:

Fix up convert.c generation

- Use a temporary file for convert.c to reduce likelihood of an interrupted
  build resulting in bad code being written to convert.c
- Truncate the file instead of appending to it to ensure that the file being
  touched will not result in duplicate declarations/definitions from
  kern_acct.c if/when kern_acct.c changes.

git-svn-id: svn://svn.freebsd.org/base/stable/10@291763 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f