qemu: disable capstone for 32-bit mips with multilib

When build lib32-qemu for qemumips with multilib:

  require conf/multilib.conf
  MACHINE = "qemumips64"
  MULTILIBS = "multilib:lib32"
  DEFAULTTUNE_virtclass-multilib-lib32 = "mips"

it fails to compile capstone:

|  CC      arch/AArch64/AArch64InstPrinter.o
|  {standard input}: Assembler messages:
|  {standard input}:36033: Error: branch out of range
|  {standard input}:36257: Error: branch out of range

Disable capstone for mips o32 in this situation as a workround.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>