libbacktrace: change PC variables from uint64_t to uintptr_t

	Change variables holding PC values from uint64_t to uintptr_t.
	Patch by Björn Schäpers.
	* dwarf.c (struct function_addrs): Change low and high fields to
	uintptr_t.
	(struct unit_addrs): Likewise.
	(resolve_addr_index): Change address parameter to uintptr_t*.
	(add_unit_addr): Change lowpc and highpc parameters to uintptr_t.
	(add_function_range): Likewise.
	(struct pcrange): Change lowpc and highpc fields to uintptr_t.
	(add_low_high_range): Change add_range lowpc and highpc parameters
	to uintptr_t.
	(add_ranges_from_ranges): Likewise.
	(add_ranges_from_rnglists): Likewise.
	(add_low_high_range): Chnage lowpc and highpc variables to
	uintpr_t.
	(add_ranges_from_rnglists): Change some local variables to
	uintptr_t.
	(add_ranges_from_ranges): Change base parameter to uintptr_t.
	(add_ranges_from_rnglists): Likewise.
	(read_function_entry): Likewise.
	(resolve_addr_index): Add explicit casts to uintptr_t.
	(update_pcrange): Likewise.
	(add_ranges_from_ranges): Likewise.
	(add_ranges_from_rnglists): Likewise.
	(read_function_entry): Likewise.