The warning about discarded sections in elf_link_input_bfd doesn't
belong there since the code is dealing with symbols. Multiple symbols
in a discarded section will result in multiple identical warnings
about the section. Move the warning to a new function in ldlang.c.
The patch also tidies the warning quoting of section and file names,
consistently using `%pA' and `%pB'. I'm no stickler for one style of
section and file name quoting, but they ought to be consistent within
a warning, eg. see the first one fixed in ldlang.c, and when a warning
is emitted for multiple targets they all ought to use exactly the same
format string to reduce translation work. elf64-ppc.c loses the
build_one_stub errors since we won't get there before hitting the
fatal errors in size_one_stub.