Use a symbol flag bit to mark linker defined symbols

Use a symbol flag bit to mark linker defined symbols

Trying to use the SEC_LINKER_CREATED section flag to determine whether
a symbol is linker defined fails to work on targets like alpha that
define special SEC_COMMON sections.  These might contain symbols that
originated in an object file.

include/
	* bfdlink.h (struct bfd_link_hash_entry): Comment non_ir_ref.  Add
	linker_def.
bfd/
	* elflink.c (_bfd_elf_define_linkage_sym): Set linker_def.
	* linker.c (_bfd_generic_link_add_one_symbol): Clear linker_def
	for CDEF, DEF, DEFW, COM.
ld/
	* ldexp.c (exp_fold_tree_1 <etree_provide>): Test linker_def.
ld/testsuite/
	* ld-powerpc/sdabase.s,
	* ld-powerpc/sdabase.t,
	* ld-powerpc/sdabase.d: New test.
	* ld-powerpc/sdabase2.t,
	* ld-powerpc/sdabase2.d: New test.
	* ld-powerpc/powerpc.exp: Run them.