Makefile: Capstone: Add support for cross compile ranlib

When cross compiling QEMU for Windows we need to specify the cross
version of ranlib to avoid build errors when building capstone. This
patch ensures we use the same cross prefix on ranlib as other toolchain
components.

- Fedora23 mingw
- RHEL-7.2 with mingw packages from epel:

   LINK    qemu-img.exe
 build-win64/capstone/capstone.lib: error adding symbols: Archive has no
index; run ranlib to add one
 collect2: error: ld returned 1 exit status

$ x86_64-w64-mingw32-ar --version
GNU ar (GNU Binutils) 2.25

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <e457d4e906dceea4de6c3431813a06b137c1ab9c.1510103351.git.alistair.francis@xilinx.com>
Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>