gdb: convert "set debug dwarf-read" to new style

Add dwarf_read_debug_printf and dwarf_read_debug_printf_v macros and use
them throughout dwarf2/read.c.  The second one is used for "verbose"
prints, when the value of "set debug dwarf-read" is >= 2.

gdb/ChangeLog:

	* dwarf2/read.c (dwarf_read_debug_printf,
	dwarf_read_debug_printf_v): New macros, use throughout the file.

Change-Id: I694da69da2e1f2caa4c27a421a975790636411e2