gas: xtensa: rewrite xg_relax_trampoline

gas: xtensa: rewrite xg_relax_trampoline

Replace linked list of trampoline frags with an ordered array, so that
instead of indexing fixups trampolines could be indexed. Keep each array
in the trampoline_seg structure, so there's no need to rebuild it for
every new processed segment. Don't run relaxation for each trampoline
frag, instead run it for each fixup in the current segment that needs
relaxation at the beginning of each relaxation pass. This way the
complexity of this process drops from about O(n^2 * m) to about
O(log n * m), where n is the number of trampoline frags and m is the
number of fixups that need relaxation in the segment.

gas/
2017-11-27  Max Filippov  <jcmvbkbc@gmail.com>

	* config/tc-xtensa.c (trampoline_index): New structure.
	(trampoline_seg): Replace trampoline list with trampoline index.
	(xg_find_trampoline, xg_add_trampoline_to_index)
	(xg_remove_trampoline_from_index, xg_add_trampoline_to_seg)
	(xg_is_trampoline_frag_full, xg_get_fulcrum)
	(xg_find_best_trampoline, xg_relax_fixup, xg_relax_fixups)
	(xg_is_relaxable_fixup): New functions.
	(J_MARGIN): New macro.
	(xtensa_create_trampoline_frag): Use xg_add_trampoline_to_seg
	instead of open-coded addition to the linked list.
	(dump_trampolines): Iterate through the trampoline_seg::index.
	(cached_fixupS, cached_fixup, fixup_cacheS, fixup_cache)
	(fixup_order, xtensa_make_cached_fixup)
	(xtensa_realloc_fixup_cache, xtensa_cache_relaxable_fixups)
	(xtensa_find_first_cached_fixup, xtensa_delete_cached_fixup)
	(xtensa_add_cached_fixup, check_and_update_trampolines): Remove
	definitions.
	(xg_relax_trampoline): Extract logic into separate functions,
	replace body with a call to xg_relax_fixups.
	(search_trampolines): Replace search in linked list with search
	in index. Change data type of address-tracking variables from
	int to offsetT. Replace abs with labs.
	(xg_append_jump): Finish the trampoline frag if it's full.
	(add_jump_to_trampoline): Remove trampoline frag from the index
	if the frag is full.
	* config/tc-xtensa.h (xtensa_frag_type): Remove next_trampoline.
	* testsuite/gas/xtensa/trampoline.d: Adjust absolute addresses
	as the placement of trampolines has slightly changed.
	* testsuite/gas/xtensa/trampoline.s: Add _nop so that objdump
	stays in sync with instruction stream.