Add AArch64 HWCAP2_EBF16 from Linux 6.0 to bits/hwcap.h

Linux 6.0 adds a new AArch64 HWCAP2 bit, HWCAP2_EBF16.  Add this to
glibc's bits/hwcap.h.

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