[sanitizer] Correct alignment of x32 __sanitizer_siginfo

Since alignment of x32 siginfo_t is 8 bytes, not 4 bytes, use u64 in
__sanitizer_siginfo_pad to align x32 __sanitizer_siginfo to 8 bytes.

Fixes https://github.com/llvm/llvm-project/issues/62394

Differential Revision: https://reviews.llvm.org/D149309