target/sh4: Fix SUBV opcode

target/sh4: Fix SUBV opcode

The documentation says:

  SUBV Rm, Rn        Rn - Rm -> Rn, underflow -> T

The overflow / underflow can be calculated as:

  T = ((Rn ^ Rm) & (Result ^ Rn)) >> 31

However we were using the incorrect:

  T = ((Rn ^ Rm) & (Result ^ Rm)) >> 31

Fix by using the Rn register instead of Rm.

Add tests provided by Paul Cercueil.

Cc: qemu-stable@nongnu.org
Fixes: ad8d25a11f ("target-sh4: implement addv and subv using TCG")
Reported-by: Paul Cercueil <paul@crapouillou.net>
Suggested-by: Paul Cercueil <paul@crapouillou.net>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2318
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Message-Id: <20240430163125.77430-3-philmd@linaro.org>
(cherry picked from commit e88a856efd1d3c3ffa8e53da4831eff8da290808)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>