aarch64: Reimplement vmovl*/vmovn* intrinsics using __builtin_convertvector

aarch64: Reimplement vmovl*/vmovn* intrinsics using __builtin_convertvector

__builtin_convertvector seems well-suited to implementing the vmovl and
vmovn intrinsics that widen and narrow
the integer elements in a vector.

This removes some more inline assembly from the intrinsics.

gcc/
	* config/aarch64/arm_neon.h (vmovl_s8): Reimplement using
	__builtin_convertvector.
	(vmovl_s16): Likewise.
	(vmovl_s32): Likewise.
	(vmovl_u8): Likewise.
	(vmovl_u16): Likewise.
	(vmovl_u32): Likewise.
	(vmovn_s16): Likewise.
	(vmovn_s32): Likewise.
	(vmovn_s64): Likewise.
	(vmovn_u16): Likewise.
	(vmovn_u32): Likewise.
	(vmovn_u64): Likewise.