elf: Store __ehdr_start hash in elf_link_hash_table

elf: Store __ehdr_start hash in elf_link_hash_table

Since

commit 97da0e2677c4a38df2406576428ec27d1da26e7c
Author: Alan Modra <amodra@gmail.com>
Date:   Wed Jan 12 23:42:23 2022 +1030

    tweak __ehdr_start visibility and flags for check_relocs

creates __ehdr_start hash in lang_symbol_tweaks, store __ehdr_start hash
in elf_link_hash_table so that we just need to lookup it up only once.

bfd/

	* elf-bfd.h (elf_link_hash_table): Add hehdr_start.
	* elf.c (assign_file_positions_for_load_sections): Use
	hehdr_start.

ld/

	* ldelf.c (ldelf_before_allocation): Use hehdr_start for
	__ehdr_start hash.
	* ldlang.c (lang_symbol_tweaks): Store hehdr_start hash in
	hehdr_start.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>