gdb: rename dwarf2_per_objfile variables/fields to per_objfile

gdb: rename dwarf2_per_objfile variables/fields to per_objfile

While doing the psymtab-sharing patchset, I avoided renaming variables
unnecessarily to avoid adding noise to patches, but I'd like to do it
now.  Basically, we have these dwarf2 per-something structures:

  - dwarf2_per_objfile
  - dwarf2_per_bfd
  - dwarf2_per_cu_data

I named the instances of dwarf2_per_bfd `per_bfd` and most of instances
of dwarf2_per_cu_data are called `per_cu`.  Most pre-existing instances
of dwarf2_per_objfile are named `dwarf2_per_objfile`.  For consistency
with the other type, I'd like to rename them to just `per_objfile`.  The
`dwarf2_` prefix is superfluous, since it's already clear we are in
dwarf2 code.  It also helps reducing the line wrapping by saving 7
precious columns.

gdb/ChangeLog:

	* dwarf2/comp-unit.c, dwarf2/comp-unit.h, dwarf2/index-cache.c,
	dwarf2/index-cache.h, dwarf2/index-write.c,
	dwarf2/index-write.h, dwarf2/line-header.c,
	dwarf2/line-header.h, dwarf2/macro.c, dwarf2/macro.h,
	dwarf2/read.c, dwarf2/read.h: Rename struct dwarf2_per_objfile
	variables and fields from `dwarf2_per_objfile` to just
	`per_objfile` throughout.

Change-Id: I3c45cdcc561265e90df82cbd36b4b4ef2fa73aef