build-sys: fix -fsanitize=address check

Since 218bb57dd79d6843e0592c30a82ea8c1fddc74a5, the -fsanitize=address
check fails with:
config-temp/qemu-conf.c:3:20: error: integer overflow in expression [-Werror=overflow]
return INT32_MIN / -1;

Interestingly, UBSAN check doesn't produce a compile time warning.
Use a test that doesn't have compile time warnings, and make it
specific to UBSAN check.

Backports commit b9f44da2f2cdc1a1a1be5aed0c46bd7fcc69cf4a from qemu

qemu/configure

@@ -1186,23 +1186,25 @@ fi
 ##########################################
 # checks for sanitizers
 
-# we could use a simple skeleton for flags checks, but this also
-# detect the static linking issue of ubsan, see also:
-# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84285
-cat > $TMPC << EOF
-#include <stdint.h>
-int main(void) {
-  return INT32_MIN / -1;
-}
-EOF
-
 have_asan=no
 have_ubsan=no
 
 if test "$sanitizers" = "yes" ; then
+  write_c_skeleton
   if compile_prog "$CPU_CFLAGS -Werror -fsanitize=address" ""; then
       have_asan=yes
   fi
+
+  # we could use a simple skeleton for flags checks, but this also
+  # detect the static linking issue of ubsan, see also:
+  # https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84285
+  cat > $TMPC << EOF
+#include <stdlib.h>
+int main(void) {
+    void *tmp = malloc(10);
+    return *(int *)(tmp + 2);
+}
+EOF
   if compile_prog "$CPU_CFLAGS -Werror -fsanitize=undefined" ""; then
       have_ubsan=yes
   fi