target/sh4: Fix translator.c assertion failure for gUSA

target/sh4: Fix translator.c assertion failure for gUSA

The translator loop does not allow the tb_start hook to set
dc->base.is_jmp; the only hook allowed to do that is translate_insn.

Split the work between init_disas_context where we validate
the gUSA parameters, and translate_insn where we emit code.

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