arm: vst1_types_x2 ACLE intrinsics

arm: vst1_types_x2 ACLE intrinsics

This patch is part of a series of patches implementing the _xN
variants of the vst1 intrinsic for the arm port. This patch adds the
_x2 variants of the vst1 intrinsic.

ACLE documents:
https://developer.arm.com/documentation/ihi0053/latest/

ISA documents:
https://developer.arm.com/documentation/ddi0487/latest/

gcc/ChangeLog:
	* config/arm/arm_neon.h
	(vst1_u8_x2, vst1_u16_x2, vst1_u32_x2, vst1_u64_x2): New.
	(vst1_s8_x2, vst1_s16_x2, vst1_s32_x2, vst1_s64_x2): New.
	(vst1_f16_x2, vst1_f32_x2): New.
	(vst1_p8_x2, vst1_p16_x2, vst1_p64_x2): New.
	(vst1_bf16_x2): New.
	* config/arm/arm_neon_builtins.def (vst1_x2): New entries.
	* config/arm/neon.md (vst1_x2<mode>): New.

gcc/testsuite/ChangeLog:
	* gcc.target/arm/simd/vst1_base_xN_1.c: Add new tests.
	* gcc.target/arm/simd/vst1_bf16_xN_1.c: Add new tests.
	* gcc.target/arm/simd/vst1_fp16_xN_1.c: Add new tests.
	* gcc.target/arm/simd/vst1_p64_xN_1.c: Add new tests.