Tweak "standard" ARM disassembler diagnostics to fail if the target
is not "an ARM" (vs. not "an ARMV4/5"), so it makes more sense for
cores inheriting this as the "generic" disassembler.

Also, to use the Thumb2 entry instead of the original Thumb entry.
This makes it work better for both newer cores (which support those
added instructions) and for BL and BLX instructions on older cores.
(Those instructions are 32-bits, which requires curious state-aware
code to go through a 16-bit decode interface...)

Plus minor cleanups, notably to have fewer exit paths and to make
sure they all return failure codes.

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