target/i386: Fix #GP error code for INT instructions

While the (intno << shift) expression is correct for indexing the IDT based on
whether Long Mode is active, the error code itself was unchanged with AMD64,
and is still the index with 3 bits of metadata in the bottom.

Found when running a Xen unit test, all under QEMU.  The unit test objected to
being told there was an error with IDT index 256 when INT $0x80 (128) was the
problem instruction:

  ...
  Error: Unexpected fault 0x800d0802, #GP[IDT[256]]
  ...

Fixes: d2fd1af767 ("x86_64 linux user emulation")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Link: https://lore.kernel.org/r/20250312000603.3666083-1-andrew.cooper3@citrix.com
Cc: qemu-stable@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3160
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

target/i386/tcg/user/seg_helper.c

@@ -63,7 +63,7 @@ static void do_interrupt_user(CPUX86State *env, int intno, int is_int,
         cpl = env->hflags & HF_CPL_MASK;
         /* check privilege if software int */
         if (dpl < cpl) {
-            raise_exception_err(env, EXCP0D_GPF, (intno << shift) + 2);
+            raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
         }
     }