Properly check --compress-debug-sections=XXX

Properly check --compress-debug-sections=XXX

We can't check if input is ELF in copy_file since some targets may only
set xvec after bfd_check_format_matches is called.  This patch moves
this check to copy_object.

bfd/

	* elfxx-target.h (TARGET_BIG_SYM): Add BFD_COMPRESS_GABI to
	object_flags.
	(TARGET_LITTLE_SYM): Likewise.

binutils/

	* objcopy.c (do_debug_sections): Use bit patterns.
	(copy_object): Return FALSE for compress_zlib, compress_gnu_zlib
	and compress_gabi_zlib on non-ELF input.
	(copy_file): Don't check non-ELF input here.