gas: xtensa: fix trampoline placement
    
    For jumps requiring multiple trampolines trampoline placement code may
    place multiple sequential trampolines into the same frag. Don't do that.
    
    gas/
    2018-02-14  Max Filippov  <jcmvbkbc@gmail.com>
    
    	Backport from mainline
    	2018-02-13  Max Filippov  <jcmvbkbc@gmail.com>
    
    	* config/tc-xtensa.c (xg_find_best_trampoline): Skip trampoline
    	frag that contains source address.