ARM: allow proper single stepping of Thumb BL and BLX instructions

ARM: allow proper single stepping of Thumb BL and BLX instructions

Whenever an unconditional branch with the H bits set to 0b10 is met, the
offset must be combined with the offset from the following opcode and not
ignored like it is now.

A comment in evaluate_b_bl_blx_thumb() suggests that the Thumb2 decoder
would be a simpler solution.  That might be true when single-stepping of
Thumb2 code is implemented.  But for now this appears to be the simplest
solution to fix Thumb1 support.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>