target/sh4: optimize gen_write_sr using extract op

This doesn't change the generated code on x86, but optimizes it on most
RISC architectures and makes the code simpler to read.

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