Add a new is_arm_mode() predicate, and use it to replace almost
all calls to current armv4_5_mode_to_number().

Eventually those internal mode numbers should vanish... along
with their siblings in the armv7a.c file.

Remove a handful of superfluous checks ... e.g. the mode number
was just initialized, or (debug entry methods) already validated.

Move one of the macros using internal mode numbers into the only
file which uses that macro.  Make the tables manipulated with
those numbers be read-only and, where possible, static so they're
not confused with part of the generic ARM interface.

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