configure: x86: improve ebp availability check

Some compilers are extra strict about register usage in main(),
disallowing ebp in inline asm there while allowing it elsewhere.
This change makes the test better reflect actual usage.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -863,6 +863,9 @@ check_exec_crash(){
 static void sighandler(int sig){
     raise(SIGTERM);
 }
+int foo(void){
+    $code
+}
 int main(void){
     signal(SIGILL, sighandler);
     signal(SIGFPE, sighandler);
@@ -870,7 +873,7 @@ int main(void){
 #ifdef SIGBUS
     signal(SIGBUS, sighandler);
 #endif
-    { $code }
+    foo();
 }
 EOF
 }