Ptrace support for AArch64 SVE gdbserver

Add checks to detect SVE tdesc. Easiest way to do this is by checking the
size of the vector registers.

Use the common aarch64 ptrace copy functions for reading/writing registers.
A wrapper is required due to the common functions using reg_buffer_common.

gdbserver/
	* linux-aarch64-low.c (is_sve_tdesc): New function.
	(aarch64_sve_regs_copy_to_regcache): Likewise.
	(aarch64_sve_regs_copy_from_regcache):  Likewise.
	(aarch64_regs_info): Add SVE checks.
	(initialize_low_arch): Initialize SVE.