SH4: SUBV instruction not emulated properly
Description of problem:
SUBV opcode is emulated incorrectly.

The documentation says:

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

Qemu seems to perform the subtraction correctly, but will not detect an underflow.
Steps to reproduce:
```c
#include <stdio.h>

int main(void)
{
	register unsigned int a asm("r8") = 0x80000001;
	register unsigned int b asm("r9") = 0x2;
	register unsigned int c asm("r10");

	asm volatile("subv %2,%0\n"
		     "movt %1\n"
		     : "+r"(a), "=r"(c) : "r"(b) :);

	printf("Values: a=0x%x b=0x%x c=0x%x\n", a, b, c);

	return 0;
}
```
Additional information:
Tested on real hardware (SEGA Dreamcast, GCC 15.0), the program above prints:
`Values: a=0x7fffffff b=0x2 c=0x1`

Running with Qemu (and GCC 13.0), the same program prints:
`Values: a=0x7fffffff b=0x2 c=0x0`