target-sh4: move features flag after CPU_COMMON

Everything before CPU_COMMON in the structure is cleared as part of a
CPU reset. This included the features flag, which indicates whether SH4A
instructions are supported or not. As a result, a CPU reset downgraded
the CPU from an SH4A to an SH4.

Signed-off-by: Bobby Bingham <koorogi@koorogi.info>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>