ld: Add a before_plugin_all_symbols_read hook

ld: Add a before_plugin_all_symbols_read hook

Add a before_plugin_all_symbols_read hook to load symbol references from
DT_NEEDED entries, included from --copy-dt-needed-entries, before reading
plugin symbols to properly resolve plugin symbol references.

bfd/

	PR ld/28849
	* elf-bfd.h (elf_link_hash_table): Add handling_dt_needed.
	* elflink.c (_bfd_elf_merge_symbol): Don't set non_ir_ref_dynamic
	before plugin 'all symbols read' hook is called.

ld/

	PR ld/28849
	* ldelf.c (ldelf_handle_dt_needed): New function.
	(ldelf_before_plugin_all_symbols_read): Likewise.
	(ldelf_after_open): Call ldelf_handle_dt_needed.
	* ldelf.h (ldelf_before_plugin_all_symbols_read): New.
	* ldemul.c (ldemul_before_plugin_all_symbols_read): Likewise.
	* ldemul.h (ldemul_before_plugin_all_symbols_read): Likewise.
	(ld_emulation_xfer_struct): Add before_plugin_all_symbols_read.
	* ldlang.c (lang_process): Call
	ldemul_before_plugin_all_symbols_read before calling
	plugin_call_all_symbols_read.
	* emultempl/elf.em
	(gld${EMULATION_NAME}_before_plugin_all_symbols_read): New.
	(LDEMUL_BEFORE_PLUGIN_ALL_SYMBOLS_READ): New.
	* emultempl/emulation.em (ld_${EMULATION_NAME}_emulation):
	Initialize the before_plugin_all_symbols_read field.
	* testsuite/ld-plugin/lto.exp: Run PR ld/28849 tests.
	* testsuite/ld-plugin/pr28849.d: New file.
	* testsuite/ld-plugin/pr28849a.c: Likewise.
	* testsuite/ld-plugin/pr28849b.c: Likewise.