Add HWCAP_SSBS from Linux 4.20 to AArch64 bits/hwcap.h.

This patch adds the HWCAP_SSBS constant from Linux 4.20 to the AArch64
bits/hwcap.h.

Tested with build-many-glibcs.py for aarch64-linux-gnu.

	* sysdeps/unix/sysv/linux/aarch64/bits/hwcap.h (HWCAP_SSBS): New
	macro.