This patch adds support for 12-bit immediate values for Thumb-2 in ADD and
SUB instructions.  We added two new alternatives for *arm_addsi3 which
make use of two new constraints for 12-bit values.  Also we modified
costs of PLUS rtx expression.
This patch reduces size of libevas by 1788 bytes (from 464916 to
463128), and sqlite by 54 bytes (from 266156 to 266052).
Regtested with Cortex-A8 QEMU.

Ok for trunk?