Fix the handling of window spill traps by keeping cansave into account
when calculating the new CWP.

Signed-off-by: Giuseppe Musacchio <thatlemon@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200625091204.3186186-3-laurent@vivier.eu>