AArch64: use movi d0, #0 to clear SVE registers instead of mov z0.d, #0

This patch changes SVE to use Adv. SIMD movi 0 to clear SVE registers when not
in SVE streaming mode.  As the Neoverse Software Optimization guides indicate
SVE mov #0 is not a zero cost move.

When In streaming mode we continue to use SVE's mov to clear the registers.

Tests have already been updated.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_output_sve_mov_immediate): Use
	fmov for SVE zeros.