gas: xtensa: merge trampoline_frag into xtensa_frag_type

gas: xtensa: merge trampoline_frag into xtensa_frag_type

The split between fragS and trampoline_frag doesn't save much space, but
makes trampolines management much more awkward. Merge trampoline_frag
data into the xtensa_frag_type, which is a part of fragS. No functional
changes.

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

	* config/tc-xtensa.c (init_trampoline_frag): Replace pointer to
	struct trampoline_frag parameter with pointer to fragS.
	(xg_append_jump): Remove jump_around parameter.
	(struct trampoline_frag): Remove.
	(struct trampoline_seg): Change type of trampoline_list from
	struct trampoline_frag to fragS.
	(xtensa_create_trampoline_frag): Don't allocate struct
	trampoline_frag. Initialize new fragS::tc_frag_data fields.
	(dump_trampolines, xg_relax_trampoline, search_trampolines)
	(get_best_trampoline, init_trampoline_frag)
	(add_jump_to_trampoline, relax_frag_immed): Replace pointer to
	struct trampoline_frag with a pointer to fragS.
	(xg_append_jump): Remove jump_around parameter, use
	fragS::tc_frag_data.jump_around_fix instead.
	(xg_relax_trampoline, init_trampoline_frag)
	(add_jump_to_trampoline): Don't pass jump_around parameter to
	xg_append_jump.
	* config/tc-xtensa.h (struct xtensa_frag_type): Add new fields:
	needs_jump_around, next_trampoline and jump_around_fix.