xcoff dependency list for static libraries

This patch fixes fails adding library dependencies for xcoff, and
improves the error message should stat fail for an archive member.
"tmpdir/artest.a: File not found" is plainly wrong.

Fixes these fails:
powerpc-aix5.1  +FAIL: ar adding library dependencies
powerpc-aix5.2  +FAIL: ar adding library dependencies
rs6000-aix4.3.3  +FAIL: ar adding library dependencies
rs6000-aix5.1  +FAIL: ar adding library dependencies
rs6000-aix5.2  +FAIL: ar adding library dependencies

	* archive.c (bfd_ar_hdr_from_filesystem): Use bfd_set_input_error
	when stat of archive member fails.
	* coff-rs6000.c (xcoff_write_archive_contents_old),
	(xcoff_write_archive_contents_big): Likewise, and handle in-memory
	bfd.