target/m68k: add explicit single and double precision operations (part 2)

Add fsabs, fdabs, fsneg, fdneg, fsmove and fdmove.

The value is converted using the new floatx80_round() function.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-Id: <20170628204241.32106-7-laurent@vivier.eu>