target/sh4: fetch code with translator_ld

There is a small wrinkle with the gUSA instruction. The translator
effectively treats a (known) gUSA sequence as a single instruction.
For the purposes of the plugin we end up with a long multi-instruction
qemu_plugin_insn.

If the known sequence isn't detected we shall never run this
translation anyway.

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