target/hppa: Use DisasContextBase.is_jmp

Instead of returning DisasJumpType, immediately store it.
Return true in preparation for conversion to the decodetree script.

Tested-by: Helge Deller <deller@gmx.de>
Tested-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>