arm: basic support for ARMv4/ARMv4T emulation

Currently target-arm/ assumes at least ARMv5 core. Add support for
handling also ARMv4/ARMv4T. This changes the following instructions:

BX(v4T and later)

BKPT, BLX, CDP2, CLZ, LDC2, LDRD, MCRR, MCRR2, MRRC, MCRR, MRC2, MRRC,
MRRC2, PLD QADD, QDADD, QDSUB, QSUB, STRD, SMLAxy, SMLALxy, SMLAWxy,
SMULxy, SMULWxy, STC2 (v5 and later)

All instructions that are "v5TE and later" are also bound to just v5, as
that's how it was before.

This patch doesn _not_ include disabling of cp15 access and base-updated
data abort model (that will be required to emulate chips based on a
ARM7TDMI), because:
* no ARM7TDMI chips are currently emulated (or planned)
* those features aren't strictly necessary for my purposes (SA-1 core
  emulation).

All v5 models are handled as they are v5T. Internally we still have a
check if the model is a v5(T) or v5TE, but as all emulated cores are
v5TE, those two cases are simply aliased (for now).

Patch is heavily based on patch by Filip Navara <filip.navara@gmail.com>
which in turn is based on work by Ulrich Hecht <uli@suse.de> and Vincent
Sanders <vince@kyllikki.org>.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>