tcg: Add TranslationBlock.jmp_insn_offset

tcg: Add TranslationBlock.jmp_insn_offset

Stop overloading jmp_target_arg for both offset and address,
depending on TCG_TARGET_HAS_direct_jump.  Instead, add a new
field to hold the jump insn offset and always set the target
address in jmp_target_addr[].  This will allow a tcg backend
to use either direct or indirect depending on displacement.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>