Cortex-M3: remove exports and forward decls

Unneeded exports cause confusion about the module interfaces.
Make most functions static, and fix some line-too-long issues.
Delete some now-obviously-unused code.

The forward decls are just code clutter; move their references
later, after the normal declarations.  (Or vice versa.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>