tcg: Introduce set_jmp_insn_offset

tcg: Introduce set_jmp_insn_offset

Similar to the existing set_jmp_reset_offset.  Move any assert for
TCG_TARGET_HAS_direct_jump into the new function (which now cannot
be build-time).  Will be unused if TCG_TARGET_HAS_direct_jump is
constant 0, but we can't test for constant in the preprocessor,
so just mark it G_GNUC_UNUSED.

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