cxa_guard: avoid sys/syscall.h and SYS_gettid am: 45ad00158c am: f73a312a36 am: 1c0c59d816

Original change: https://android-review.googlesource.com/c/platform/external/libcxxabi/+/1729852

Change-Id: I1129d2d967968a63b5364280aaf5abee1307029a