Nios2 dynobj handling fixes

Nios2 dynobj handling fixes

A number of places in elf32-nios.c created dynamic sections but didn't
set the hash table dynobj.  That meant we could have duplicate dynamic
sections connected to a number of bfds, so size_dynamic_sections
didn't properly discard or allocate contents.

Also, the entire set of dynamic sections was created in check_relocs
on seeing GOT relocs, when only .got related sections are needed,
probably done to hide segfaults later in finish_dynamic_sections.

The patch fixes these issues and makes the assembler emit errors when
nios2 lacks the necessary pc-relative relocs for subtraction
expressions, rather than silently generating bad code.
eg. ld-elf/merge.  I've also tidied uses of elf32_nios2_hash_table and
elf_hash_table.

bfd/
	PR 20995
	* elf32-nios2.c (nios2_elf32_relocate_section): Use htab
	rather than elf32_nios2_hash_table or elf_hash_table.
	(create_got_section): Likewise.
	(nios2_elf32_finish_dynamic_symbol): Likewise.
	(nios2_elf32_adjust_dynamic_symbol): Likewise.
	(nios2_elf32_size_dynamic_sections): Likewise.
	(nios2_elf32_check_relocs): Delete dynobj, sgot, and srelgot
	vars.  Use htab equivalents directly instead.  Don't create
	all dynamic sections on needing just the GOT.  Use a goto
	rather than a fall-through with reloc test.  Ensure
	htab->dynobj is set when making dynamic sreloc section.
	(nios2_elf32_finish_dynamic_sections): Delete dynobj, use htab
	equivalent directly instead.  Don't segfault on looking for
	.dynamic when dynamic sections have not been created.  Don't
	segfault on .got.plt being discarded.
	(nios2_elf32_size_dynamic_sections): Delete plt and got vars.
	Don't set "relocs" on .rela.plt.  Do handle .sbss.  Delete
	fixme and another not so relevant comment.
	(nios2_elf_add_symbol_hook): Delete dynobj var.  If not
	already set, set hash table dynobj on creating .sbss.
gas/
	* config/tc-nios2.h (TC_FORCE_RELOCATION_SUB_LOCAL): Define.
ld/
	* testsuite/ld-elf/merge.d: xfail for nios.