gas: xtensa: implement trampoline coalescing

There is a recurring pattern in assembly files generated by a compiler
where a lot of jumps in a function are going to the same place. When
these jumps are relaxed with trampolines the assembler generates a
separate jump thread from each source.
Create an index of trampoline jump targets for each segment and see if a
jump being relaxed goes to a location from that index, in which case
replace its target with a location of existing trampoline jump that
results in the shortest path to the original target.

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

	* config/tc-xtensa.c (trampoline_chain_entry, trampoline_chain)
	(trampoline_chain_index): New structures.
	(trampoline_index): Add chain_index field.
	(xg_order_trampoline_chain_entry, xg_sort_trampoline_chain)
	(xg_find_chain_entry, xg_get_best_chain_entry)
	(xg_order_trampoline_chain, xg_get_trampoline_chain)
	(xg_find_best_eq_target, xg_add_location_to_chain)
	(xg_create_trampoline_chain, xg_get_single_symbol_slot): New
	functions.
	(xg_relax_fixups): Call xg_find_best_eq_target to adjust jump
	target to point to an existing jump. Call
	xg_create_trampoline_chain to create new jump target. Call
	xg_add_location_to_chain to add newly created trampoline jump
	to the corresponding chain.
	(add_jump_to_trampoline): Extract loop searching for a single
	slot with a symbol into a separate function, replace that code
	with a call to that function.
	(relax_frag_immed): Call xg_find_best_eq_target to adjust jump
	target to point to an existing jump.
	* testsuite/gas/xtensa/all.exp: Add trampoline-2 test.
	* testsuite/gas/xtensa/trampoline.d: Adjust absolute addresses
	as many duplicate trampoline chains are now coalesced.
	* testsuite/gas/xtensa/trampoline.s: Add _nop so that objdump
	stays in sync with instruction stream.
	* testsuite/gas/xtensa/trampoline-2.l: New test result file.
	* testsuite/gas/xtensa/trampoline-2.s: New test source file.