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.

MFC after: 1 week

tests/sys/kern/acct/Makefile

@@ -6,12 +6,13 @@ ATF_TESTS_C=	acct_test
 
 CFLAGS+=	-I${.OBJDIR}
 
-CLEANFILES+=	convert.c
+CLEANFILES+=	convert.c convert.c.tmp
 
 DPSRCS.acct_test=	convert.c
 
 convert.c: ${SRCTOP}/sys/kern/kern_acct.c
 	sed -n -e 's/log(/syslog(/g' \
-	       -e '/FLOAT_CONVERSION_START/,/FLOAT_CONVERSION_END/p' ${.ALLSRC} >>${.TARGET}
+	       -e '/FLOAT_CONVERSION_START/,/FLOAT_CONVERSION_END/p' ${.ALLSRC} >${.TARGET}.tmp
+	mv ${.TARGET}.tmp ${.TARGET}
 
 .include <bsd.test.mk>