Move some dwarf2_cu methods to new file

This moves some of the dwarf2_cu methods to a new file, dwarf2/cu.c.

gdb/ChangeLog
2021-05-17  Tom Tromey  <tom@tromey.com>

	* dwarf2/read.c (dwarf2_cu::addr_sized_int_type)
	(dwarf2_cu::start_symtab, dwarf2_cu::addr_type)
	(dwarf2_cu::dwarf2_cu): Move to cu.c.
	* dwarf2/cu.c: New file.
	* Makefile.in (COMMON_SFILES): Add dwarf2/cu.c.