Use strtab with GC and suffix merging for .strtab

Use strtab with GC and suffix merging for .strtab

This patch uses ELF strtab with GC and suffix merging support to create
ELF .strtab section.  There is some small memory overhead to use ELF
strtab:

==14928== HEAP SUMMARY:
==14928==     in use at exit: 3,276,318 bytes in 679 blocks
==14928==   total heap usage: 1,544 allocs, 865 frees, 15,259,146 bytes allocated

vs.

==14936== HEAP SUMMARY:
==14936==     in use at exit: 3,276,318 bytes in 679 blocks
==14936==   total heap usage: 1,532 allocs, 853 frees, 15,026,402 bytes allocated

when running:

./ld-new -m elf_x86_64 -o tmpdir/ld-partial.o -r ldgram.o ldlex-wrapper.o lexsup.o ldlang.o mri.o ldctor.o ldmain.o plugin.o ldwrite.o ldexp.o ldemul.o ldver.o ldmisc.o ldfile.o ldcref.o eelf_x86_64.o eelf32_x86_64.o eelf_i386.o eelf_iamcu.o ei386linux.o eelf_l1om.o eelf_k1om.o ldbuildid.o

The results are

  [32] .strtab STRTAB 0+ 3beff8 00407a 00 0 0 1

vs

  [32] .strtab STRTAB 0+ 3beff8 0041d8 00 0 0 1

It reduces the .strtab size by 350 bytes, about 2%

Saving on libc.so from glibc is much more since libc.so has many alias
symbols with the same suffix.  For x32 glibc,

 [82] .strtab STRTAB 0+ 81b348 0159e7 00 0 0 1

vs

 [82] .strtab STRTAB 0+ 81b8bc 019e72 00 0 0 1

It reduces the .strtab size by 17547 bytes, about 16%.

bfd/

	PR gas/18451
	* elf-bfd.h (elf_sym_strtab): New.
	(elf_link_hash_table): Add strtabcount, strtabsize and
	strtab.
	(_bfd_elf_stringtab_init): Removed.
	* elf.c (_bfd_elf_stringtab_init): Removed.
	(_bfd_elf_compute_section_file_positions): Replace
	bfd_strtab_hash/_bfd_elf_stringtab_init/_bfd_stringtab_free/
	_bfd_stringtab_size with
	elf_strtab_hash/_bfd_elf_strtab_init/_bfd_elf_strtab_free/
	_bfd_elf_strtab_size.  Use _bfd_elf_strtab_add,
	_bfd_elf_strtab_finalize and _bfd_elf_strtab_offset to get
	st_name.
	(swap_out_syms): Likewise.
	* elflink.c (elf_final_link_info): Replace bfd_strtab_hash
	with elf_strtab_hash.  Remove symbuf, symbuf_count,
	symbuf_size and shndxbuf_size.
	(elf_link_flush_output_syms): Removed.
	(elf_link_output_sym): Renamed to ...
	(elf_link_output_symstrtab): This.  Replace _bfd_stringtab_add
	with _bfd_elf_strtab_add.  Don't flush symbols to the file nor
	swap out symbols.
	(elf_link_swap_symbols_out): New.
	(elf_link_output_extsym): Replace elf_link_output_sym with
	elf_link_output_symstrtab.
	(elf_link_input_bfd): Likewise.
	(elf_final_link_free): Replace _bfd_stringtab_free with
	_bfd_elf_strtab_free.  Remove symbuf.
	(bfd_elf_final_link): Replace _bfd_elf_stringtab_init with
	_bfd_elf_strtab_init.  Don't set symbuf, symbuf_count,
	symbuf_size nor shndxbuf_size.  Initialize strtabsize and
	strtab.  Initialize symshndxbuf to -1 when number of sections
	>= 64K.  Replace elf_link_output_sym/elf_link_output_sym with
	elf_link_output_symstrtab/elf_link_output_symstrtab. Don't
	call elf_link_flush_output_syms.  Call _bfd_elf_strtab_finalize
	and elf_link_swap_symbols_out.  Replace _bfd_stringtab_size
	and _bfd_stringtab_emit with _bfd_elf_strtab_size and
	_bfd_elf_strtab_emit.

gas/testsuite/

	PR gas/18451
	* gas/elf/elf.exp: Run strtab.
	* gas/elf/strtab.d: New file.
	* gas/elf/strtab.s: Likewise.

ld/testsuite/

	PR gas/18451
	* ld-elf/strtab.d: New file.
	* ld-elf/strtab.s: Likewise.