AArch64: Implement missing p128<->f64 reinterpret intrinsics

AArch64: Implement missing p128<->f64 reinterpret intrinsics

This patch implements the missing reinterprets to and from poly128_t and
float64x2_t.
I've plugged in the appropriate testing in the advsimd-intrinsics.exp
too.

Bootstrapped and tested on aarch64-none-linux-gnu.
Tested advsimd-intrinsics.exp on arm-none-eabi too to make sure arm
testing isn't affected.

gcc/
	PR target/71233
	* config/aarch64/arm_neon.h (vreinterpretq_f64_p128,
	vreinterpretq_p128_f64): Define.

gcc/testsuite/
	PR target/71233
	* gcc.target/aarch64/advsimd-intrinsics/arm-neon-ref.h
	(clean_results): Add float64x2_t cleanup.
	(DECL_VARIABLE_128BITS_VARIANTS): Add float64x2_t variable.
	* gcc.target/aarch64/advsimd-intrinsics/vreinterpret_p128.c: Add
	testing of vreinterpretq_f64_p128, vreinterpretq_p128_f64.