gdb/dwarf: Introduce dwarf2_per_cu_int_type function

gdb/dwarf: Introduce dwarf2_per_cu_int_type function

This is a minor refactor in preparation for the next commit.  Splits
the core of dwarf2_per_cu_addr_sized_int_type out into a separate
function.  There should be no user visible changes after this commit.

gdb/ChangeLog:

	* dwarf2read.c (dwarf2_per_cu_int_type): New function, takes most
	of its implementation from...
	(dwarf2_per_cu_addr_sized_int_type): ...here, which now just calls
	the new function.

Change-Id: I8b849dd338012ec033b3f0a57d65cec0d7a3bd97