PR25882, .gnu.attributes are not checked for shared libraries

PR25882, .gnu.attributes are not checked for shared libraries

This allows backend merge_private_bfd_data to examine shared library
e_flags and/or .gnu.attributes.  ARM and PowerPC have done so when
using ld.gold for a long time.

(The tic6x change below is dead code due to the earlier FIXME,
but this is probably one of the changes needed there.)

	PR 25882
bfd/
	* elf32-tic6x.c (elf32_tic6x_merge_attributes): Don't transfer
	Tag_ABI_PIC or Tag_ABI_PID from dynamic objects to the output.
ld/
	* ldlang.c (lang_check): Call bfd_merge_private_bfd_data for
	shared libraries.