Add HWCAP2_SVE_EBF16 from Linux 6.1 to AArch64 bits/hwcap.h

Linux 6.1 adds a new AArch64 HWCAP2 value HWCAP2_SVE_EBF16; add it to
the corresponding bits/hwcap.h.

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