target/sh4: Add missing FPSCR.PR == 0 checks

Both frchg and fschg require PR == 0, otherwise undefined_operation.

Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Message-Id: <20170718200255.31647-26-rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
