We don't really need to keep S and B separate for debug source purposes
and there shouldn't be source references in WORKDIR that isn't S and B
either.
Separating these out simplifies the shared-work directory handling for
gcc and should also help fix external source usage. Therefore handle
S and B in DEBUG_PREFIX_MAP separately and clean up other code.
Indentation is reduced here as it is introduced on every compiler
commandline so minimising it is helpful.
(From OE-Core rev: c39b5020b8705d17e3745c41e38d0f99a1ac94cf)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>