gas: use literals/const16 for xtensa loop relaxation

Loop opcode relaxation that uses addi/addmi doesn't work well with other
relaxations that may cause code movement. Instead of encoding fixed loop
end offset in the relaxed sequence use l32r or a pair of const16 to load
loop end address. This way the address of the loop end gets a relocation
record and it gets updated appropriately.

gas/
2019-04-03  Max Filippov  <jcmvbkbc@gmail.com>

	* config/tc-xtensa.c (convert_frag_immed): Drop
	convert_frag_immed_finish_loop invocation.
	(convert_frag_immed_finish_loop): Drop declaration and
	definition.
	* config/xtensa-relax.c (widen_spec_list): Replace loop
	widening that uses addi/addmi with widening that uses l32r
	and const16.