gas: xtensa: extract jump assembling for trampolines

gas: xtensa: extract jump assembling for trampolines

init_trampoline_frag, add_jump_to_trampoline and xg_relax_trampoline add
a jump to the end of a trampoline frag. Extract it into a separate
funciton and use it in all these places. No functional changes.

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

	* config/tc-xtensa.c (xg_append_jump): New function.
	(xg_relax_trampoline, init_trampoline_frag)
	(add_jump_to_trampoline): Replace trampoline jump assembling
	code with a call to xg_append_jump.