The code to not set empty hardware BPs/WPs on new threads will only work
if the newly allocated struct is empty.

Ensure the structure is created zeroed.

This patch removes the remaining racy behaviour on gdb.threads tests
when run on AArch64 Ubuntu.

gdb/ChangeLog:

	* nat/aarch64-linux.c (aarch64_linux_new_thread): Replace XNEW
	with XCNEW.