aarch64: Add a %Z operand modifier for SVE registers

This patch adds a %Z operand modifier that prints registers as SVE z
registers.  The SME patches need this, but so do Tamar's patches.
I'm separating this out to unblock those.

We should probably document the [wxbhsdqZ] modifiers as
user-facing, but doing that for all of them is a separate patch.

gcc/
	* config/aarch64/aarch64.cc (aarch64_print_operand): Add a %Z
	modifier for SVE registers.