target/sh4: trap unaligned accesses

SH4 requires that memory accesses are naturally aligned, except for the
SH4-A movua.l instructions which can do unaligned loads.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>