tools/tests/x86_emulator: fix 'shifting a negative signed value is undefined'

clang-21 refuses to compile this:
```
test_x86_emulator.c:1164:24: error: shifting a negative signed value is undefined [-Werror,-Wshift-negative-value]
 1164 |     regs.r8     = (-1L << 40) + 1;
      |                    ~~~ ^
1 error generated.
```

The shift could also exceed the width of the type on some systems.
I confirmed that both the old and the new code would produce the same value
with GCC on x86-64: `0xffffff0000000001`.

Signed-off-by: Edwin Török <edwin.torok@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

tools/tests/x86_emulator/test_x86_emulator.c

@@ -1161,7 +1161,7 @@ int main(int argc, char **argv)
     instr[0] = 0x4d; instr[1] = 0x0f; instr[2] = 0xbb; instr[3] = 0x03;
     regs.eflags = EFLAGS_ALWAYS_SET;
     regs.rip    = (unsigned long)&instr[0];
-    regs.r8     = (-1L << 40) + 1;
+    regs.r8     = (~0UL << 40) + 1;
     regs.r11    = (unsigned long)(res + (1L << 35));
     rc = x86_emulate(&ctxt, &emulops);
     if ( (rc != X86EMUL_OKAY) ||