This constifies the argument to make_tempname and make_tempdir,
removing some casts.

I initially thought that this obsoleted the allocation in
write_archive, but write_archive closes the BFD before using the name,
so this appears not to be the case.

binutils/ChangeLog
2019-09-11  Tom Tromey  <tom@tromey.com>

	* objcopy.c (copy_archive): Update.
	* bucomm.h (make_tempname, make_tempdir): Make argument const.
	* bucomm.c (make_tempname, make_tempdir): Make argument const.