Add SVE-accelerated Vec::retain_mut for aarch64

[ywxt]

The PR adds SVE support for specified width types(8, 16, 32 and 64 bits) in Vec::retain_mut. Due to pointer provenance being stripped by intrinsics) here it has to use inline asm instead of sve intrinsics.

1. retain half (ns/iter)

Elements	u32 SVE	u32 scalar	Change	u64 SVE	u64 scalar	Change
4	10.32	12.36	/	10.25	11.83	/
8	12.93	15.08	/	13.40	14.74	/
16	18.66	19.46	/	19.08	18.89	/
32	31.84	31.12	/	31.78	31.26	/
64	32.99	59.17	-44.2%	59.86	59.44	/
1,000	471.51	811.71	-41.9%	820.98	827.16	/
10,000	4,660	7,990	-41.7%	5,836	8,242	-29.2%
100,000	46,414	79,608	-41.7%	57,561	82,861	-30.5%

2. retain whole

Elements	u32 SVE	u32 scalar	Change	u64 SVE	u64 scalar	Change
4	3.46	3.11	/	3.45	3.45	/
8	4.90	4.49	/	4.83	6.22	/
16	8.44	8.14	/	8.44	11.74	/
32	15.83	15.51	/	15.83	22.79	/
64	21.57	30.66	-29.6%	30.72	44.89	/
1,000	358.53	483.33	-25.8%	469.18	696.43	/
10,000	3,127	4,745	-34.1%	5,082	6,912	-26.5%
100,000	32,509	49,501	-34.3%	49,484	79,749	-38.0%

r? @Amanieu