[PowerPC] Fix build error in PPCAsmPrinter.cpp (NFC)

/Users/jiefu/llvm-project/llvm/lib/Target/PowerPC/PPCAsmPrinter.cpp:2753:34: error: cannot initialize a parameter of type 'int64_t' (aka 'long long') with an rvalue of type 'thread::id' (aka '_opaque_pthread_t *')
              "_" + llvm::itostr(llvm::this_thread::get_id()) + "_" +
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/jiefu/llvm-project/llvm/include/llvm/ADT/StringExtras.h:315:35: note: passing argument to parameter 'X' here
inline std::string itostr(int64_t X) {
                                  ^
1 error generated.