simplify XScale debug handler installation

Load the XScale debug handler from the read-only data section
instead of from a separate file that can get lost or garbaged.
This eliminates installation and versioning issues, and also
speeds up reset handling a bit.

Plus some minor bits of cleanup related to loading that handler:
comments about just what this handler does, and check fault codes
while writing it into the mini-icache.

The only behavioral changes should be cleaner failure modes after
errors during handler loading, and being a bit faster.

NOTE:  presumes GNU assembly syntax, with ".incbin"; and ELF,
because of the syntax of the ".size" directive.

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