x86emul: correct stub invocation constraints again

While the hypervisor side of commit cd91ab08ea ("x86emul: correct stub
invocation constraints") was fine, the tools side triggered a bogus
error with old gcc (4.3 and 4.4 at least). Use a slightly less
appropriate variant instead, proven to be good enough to not
re-introduce the original problem: Which of the addresses is actually
used doesn't matter much as long as the compiler can't prove that the
two pointers don't alias one another.

Reported-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -901,7 +901,7 @@ do{ asm volatile (
 # define invoke_stub(pre, post, constraints...)                         \
     asm volatile ( pre "\n\tcall *%[stub]\n\t" post                     \
                    : constraints, [stub] "rm" (stub.func),              \
-                     "m" (*(uint8_t(*)[MAX_INST_LEN + 1])stub.buf) )
+                     "m" (*(typeof(stub.buf) *)stub.addr) )
 #endif
 
 #define emulate_stub(dst, src...) do {                                  \