target.h: Expands complete_target_initialization and add_target comments.

Expand a bit the comments to answer some questions I had when looking
at why a target of mine would not have some default methods set.

gdb/ChangeLog:

        * target.h (complete_target_initialization, add_target):
        Add comment.
read_cutu_die_from_dwo: Misc minor cleanups.

	* dwarf2read.c (read_cutu_die_from_dwo): Fix function comment.
	Remove unused local comp_dir_attr.  Assert exactly one of
	stub_comp_unit_die, stub_comp_dir is non-NULL.
* dwarf2read.c (read_str_index): Rename local dwo_name to objf_name.
Better overflow checking for powerpc64 relocations

R_PPC64_ADDR16 is used in three contexts:
- .short data relocation
- 16-bit signed insn fields, eg. addi
- 16-bit unsigned insn fields, eg. ori
In the first case we want to allow both signed and unsigned 16-bit
values, the latter two ought to error if the field exceeds the range
of values allowed for 16-bit signed and unsigned integers
respectively.  These conflicting requirements meant that ld had to
choose the least restrictive overflow checks, and thus it is possible
to construct testcases where an addi field overflows but is not
reported by ld.  Many relocations dealing with 16-bit insn fields have
this problem.  What's more, some relocations that are only ever used
for signed fields of instructions woodenly copied the lax overflow
checking of R_PPC64_ADDR16.

bfd/
	* elf64-ppc.c (ppc64_elf_howto_raw): Use complain_overflow_signed
	for R_PPC64_ADDR14, R_PPC64_ADDR14_BRTAKEN, R_PPC64_ADDR14_BRNTAKEN,
	R_PPC64_SECTOFF, R_PPC64_ADDR16_DS, R_PPC64_SECTOFF_DS,
	R_PPC64_REL16 entries.  Use complain_overflow_dont for R_PPC64_TOC.
	(ppc64_elf_relocate_section): Modify overflow test for 16-bit
	fields in instructions to signed/unsigned according to whether
	the field takes a signed or unsigned value.
gold/
	* powerpc.cc (Powerpc_relocate_functions::Overflow_check): Add
	CHECK_UNSIGNED, CHECK_LOW_INSN, CHECK_HIGH_INSN.
	(Powerpc_relocate_functions::has_overflow_unsigned): New function.
	(Powerpc_relocate_functions::has_overflow_bitfield,
	overflowed): Use the above.
	(Target_powerpc::Relocate::relocate): Correct overflow checking
	for a number of relocations.  Modify overflow test for 16-bit
	fields in instructions to signed/unsigned according to whether
	the field takes a signed or unsigned value.