gas: xtensa: make trampolines relaxation work with jumps in slots other than 0

gas: xtensa: make trampolines relaxation work with jumps in slots other than 0

add_jump_to_trampoline assumes that jump instruction is in slot 0,
when it's in other slot that results in fixup that references NULL symbol,
which results in segfault later in xtensa_make_cached_fixup.
Search for the non-NULL symbol in the tc_frag_data.slot_symbols and check
that there's exactly one such slot.

xtensa_relax_frag for RELAX_TRAMPOLINE reassigns fixup from the original
instruction with jump to generated jump in the trampoline frag, but does not
fix its fx_r_type or fx_size. That results in "undecodable fix" or
"fixup not contained within frag" error messages during relaxation.
Fix both these fields.

gas/
2017-03-22  Max Filippov  <jcmvbkbc@gmail.com>

	* config/tc-xtensa.c (xtensa_relax_frag): Change fx_size of the
	reassigned fixup to size of jump instruction (3) and fx_r_type
	to BFD_RELOC_XTENSA_SLOT0_OP, as there's only one slot.
	(add_jump_to_trampoline): Search
	origfrag->tc_frag_data.slot_symbols for the slot with non-NULL
	symbol and use that slot instead of slot 0.