gas: target string hash tables

This allocates entries added to the string hash tables on the notes
obstack, so that at least those do not leak.  A followup patch will
switch over the str_hash allocation to notes_calloc, which is why I
haven't implemented deleting all the target string hash tables.

	* config/obj-coff-seh.c (get_pxdata_name, alloc_pxdata_item): Use
	notes obstack for string hash table entries.
	* config/tc-alpha.c (get_alpha_reloc_tag, md_begin): Likewise.
	* config/tc-h8300.c (md_begin): Likewise.
	* config/tc-ia64.c (dot_rot, dot_pred_rel, dot_alias): Likewise.
	* config/tc-nds32.c (nds32_relax_hint): Likewise.
	* config/tc-riscv.c (riscv_init_csr_hash): Likewise.
	* config/tc-score.c (s3_insert_reg): Likewise.
	(s3_build_score_ops_hsh, s3_build_dependency_insn_hsh): Likewise.
	* config/tc-score7.c (s7_build_score_ops_hsh): Likewise.
	(s7_build_dependency_insn_hsh): Likewise.
	* config/tc-tic4x.c (tic4x_asg): Likewise.