* Seal away private thread and kernel definitions and declarations
into the internal headers in order to better hide internal structure.

* Add a thread-common.c file that keeps shared functions together.
List functions aren't messed with since that's about to be changed to
different ones.

* It is necessary to modify some ARM/PP stuff since GCC was complaining
about constant pool distance and I would rather not force dump it. Just
bl the cache calls in the startup and exit code and let it use veneers
if it must.

* Clean up redundant #includes in relevant areas and reorganize them.

* Expunge useless and dangerous stuff like remove_thread().

Change-Id: I6e22932fad61a9fac30fd1363c071074ee7ab382

This dir is suitable for stuff that doesn't fit the target tree, e.g. because
it also builds on hosted or otherwise. It also has a generic subfolder for
fallback C implementations so that not all archs need to provide asm files.

SOURCES should only contain "foo.c" where foo.c includes the specific
<arch>/foo.c files from the subdirs using the preprocessor. This way automatic
selection of asm versions or generic C verion is possible.

For the start, the thread support files are moved, since ASM threads can
be used on hosted platforms as well. Since core_sleep() remains platform
specific it's moved to the corresponding system.h headers.

Change-Id: Iebff272f3407a6eaafeb7656ceb0ae9eca3f7cb9