i386: split svm_helper into sysemu and stub-only user

i386: split svm_helper into sysemu and stub-only user

For now we just copy over the previous user stubs, but really,

everything that requires s->cpl == 0 should be impossible
to trigger from user-mode emulation.

Later on we should add a check that asserts this easily f.e.:

static bool check_cpl0(DisasContext *s)
{
     int cpl = s->cpl;
 #ifdef CONFIG_USER_ONLY
     assert(cpl == 3);
 #endif
     if (cpl != 0) {
         gen_exception(s, EXCP0D_GPF, s->pc_start - s->cs_base);
         return false;
     }
     return true;
}

Signed-off-by: Claudio Fontana <cfontana@suse.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20210322132800.7470-17-cfontana@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>