configure: disable clang -Wstring-plus-int warning

configure: disable clang -Wstring-plus-int warning

Some versions of clang will warn about adding integers to strings:

disas/i386.c:4753:23: error: adding 'char' to a string does not append
      to the string [-Werror,-Wstring-plus-int]
      oappend ("%es:" + intel_syntax);
               ~~~~~~~^~~~~~~~~~~~~~
disas/i386.c:4753:23: note: use array indexing to silence this warning
      oappend ("%es:" + intel_syntax);
                      ^
               &      [             ]

disas/i386.c uses this idiom to to skip a "%" prefix if using intel
rather than AT&T syntax. This seems like a reasonable  thing to do,
and I don't think anybody contributing to QEMU is likely to believe
that '+' is a string concatenation operator in C, so just disable
-Wstring-plus-int.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>