In preparation for compiling tcg/ only once, eliminate
the all_helpers array.  Instantiate the info structs for
the generic helpers in accel/tcg/, and the structs for
the target-specific helpers in each translate.c.

Since we don't see all of the info structs at startup,
initialize at first use, using g_once_init_* to make
sure we don't race while doing so.

Reviewed-by: Anton Johansson <anjo@rev.ng>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>