build: fix __int128_t redefinition with Clang on MSYS2

[moleium]

Fixes a build failure on Windows when using the MSYS2 clang64 toolchain.

The compiler already defines __int128_t which caused a typedef redefinition error. This PR updates the preprocessor directive in qemu/include/qemu/int128.h to avoid this redefinition for all Clang compilers.

 unicorn/qemu/include/qemu/int128.h:150:16: error: typedef redefinition with different types ('Int128' (aka 'struct Int128') vs '__int128')
  150 | typedef Int128 __int128_t;
      |                ^
1 error generated.