Code cleanup: Split dwarf2_ranges_read to a callback

Code cleanup: Split dwarf2_ranges_read to a callback

DWARF-5 has .debug_rnglists which is somehow similar to .debug_ranges.

This patch converts dwarf2_ranges_read to dwarf2_ranges_process which can work
with both DWARF kinds of range lists through a callback.

It also simplifies dwarf2_record_block_ranges which can benefit from it.

gdb/ChangeLog
2017-02-20  Jan Kratochvil  <jan.kratochvil@redhat.com>

	* dwarf2read.c (dwarf2_ranges_process): New function from
	dwarf2_ranges_read.
	(dwarf2_ranges_read, dwarf2_record_block_ranges): Use
	dwarf2_ranges_process.