DEFAULT_BUFFERSIZE

There isn't any reason to think that a particular buffer size is
ideal in bfd, so let's just not define it.

	* libbfd-in.h (DEFAULT_BUFFERSIZE): Don't define.
	* libbfd.h: Regenerate.
	* archive.c (AR_WRITE_BUFFERSIZE): Substitute value.
	* vms-lib.c (_bfd_vms_lib_write_archive_contents): Likewise.
	* coff-rs6000.c (do_copy): Likewise, and use sizeof.