aarch64/sme2: Teach binutils/BFD about the NT_ARM_ZT register set

The Scalable Matrix Extension v2 (SME2) defines a new register, ZT0, that
the Linux Kernel handles through a new NT_ARM_ZT register set.

Teach binutils/BFD about it so that gdb can make use of it for reading
and writing core files.  This also enables readelf/objdump to show the
correct identification for the NT_ARM_ZT register set.

Validated under Fast Models.