Fix address violation when attempting to read a corrupt field in a COFF archive heade...

Fix address violation when attempting to read a corrupt field in a COFF archive header structure.

	PR 21786
	* coff-rs6000.c (_bfd_strntol): New function.
	(_bfd_strntoll): New function.
	(GET_VALUE_IN_FIELD): New macro.
	(EQ_VALUE_IN_FIELD): new macro.
	(_bfd_xcoff_slurp_armap): Use new macros.
	(_bfd_xcoff_archive_p): Likewise.
	(_bfd_xcoff_read_ar_hdr): Likewise.
	(_bfd_xcoff_openr_next_archived_file): Likewise.
	(_bfd_xcoff_stat_arch_elt): Likewise.