Use field_string in more places

Use field_string in more places

This replaces uses of field_fmt with a "%s" format string to use
field_string instead.  Also, one use of "%9lx" is replaced with a call
to phex_nz; the '9' is dropped as it is implicit in the field width.

2019-07-15  Tom Tromey  <tromey@adacore.com>

	* mi/mi-main.c (list_available_thread_groups): Use field_string.
	* mi/mi-interp.c (mi_memory_changed): Use field_string.
	* target.c (flash_erase_command): Use field_string.
	* infrun.c (print_signal_received_reason): Use field_string.
	* i386-tdep.c (i386_mpx_print_bounds): Use field_string.
	* breakpoint.c (maybe_print_thread_hit_breakpoint): Use
	field_string.
	* ada-tasks.c (print_ada_task_info): Use field_string.