[hwasan] Replace _Unwind_Word with uintptr_t

GCC introduced `__attribute__((mode(unwind_word)))` to work around
Cell Broadband Engine SPU (which was removed from GCC in 2019-09),
which is irrelevant to hwasan.
_Unwind_GetGR/_Unwind_GetCFA from llvm-project/libunwind don't use unwind_word.

Using _Unwind_Word can lead to build failures if libunwind's unwind.h is
preferred over unwind.h in the Clang resource directory (e.g. built with GCC).