Add NEON support for is_ascii and eq_ignore_ascii_case

[ywxt]

Results on my aarch64 machine.

1. is_ascii

1.1 Raw Data (ns/iter, case00_libcore only — the path that calls bytes.is_ascii())

Input	Baseline (NEON off)	Optimized (NEON on)	Change
long	19.69	6.46	-67.2%
unaligned_both_long	22.60	8.92	-60.5%
unaligned_head_long	20.62	8.81	-57.3%
unaligned_tail_long	21.15	8.68	-59.0%
medium	3.10	2.92	-5.8%
short	3.77	3.81	+1.1%
unaligned_both_medium	2.59	2.83	+9.3%
unaligned_head_medium	2.95	5.00	+69.5%
unaligned_tail_medium	2.37	2.96	+24.9%

1.2 Grouped Comparison (case00_libcore — the path affected by the change)

Group	Baseline sum (ns)	Optimized sum (ns)	Change
LONG (~350B, 4 items)	84.06	32.87	-60.9%
MEDIUM/SHORT (~16–32B, 5 items)	14.78	17.52	+18.5%
All 9 items	98.84	50.39	-49.0%

2. eq_ignore_ascii_case

2.1 Raw Data (ns/iter)

Bench	Baseline (NEON off)	Optimized (NEON on)	Change
bench_large_str_eq	6235.89	546.61	-91.2%
bench_medium_str_eq	780.81	68.05	-91.3%
bench_medium_str_tail_mismatch	786.84	65.76	-91.6%
bench_str_31_bytes_eq	35.87	3.98	-88.9%
bench_str_17_bytes_eq	19.62	4.05	-79.4%
bench_medium_str_early_mismatch	1.32	2.42	+0.75 ns
bench_str_of_8_bytes_eq	9.20	9.20	0.0%
bench_str_under_8_bytes_eq	3.49	3.49	0.0%

2.2 Grouped Comparison

Group	Scenario	Baseline	Optimized	Change
≥16B equal	17B / 31B / MEDIUM / LARGE	19.6–6236 ns	4.0–547 ns	-79% ~ -91%
≥16B tail mismatch	MEDIUM, last byte differs	786.84 ns	65.76 ns	-91.6%
≥16B early mismatch	MEDIUM, first byte differs	1.32 ns	2.42 ns	+0.75 ns
<16B	8B / <8B equal	3.49–9.20 ns	3.49–9.20 ns	no change

r? @Amanieu

[Amanieu]

@bors r+

[rust-bors]

📌 Commit ccbf7c6 has been approved by Amanieu

It is now in the queue for this repository.

[rust-bors]

⌛ Testing commit ccbf7c6 with merge 37c88a8...

Workflow: https://github.com/rust-lang/rust/actions/runs/30779241503

[Zalathar]

@bors yield (to enclosing rollup)

[rust-bors]

Auto build was cancelled. Cancelled workflows:

• https://github.com/rust-lang/rust/actions/runs/30779241503

The next pull request likely to be tested is #160411.