Neon impl of ChaCha20 (better size & perf)

[daverodgman]

Description

Neon implementation of ChaCha20 which, depending on how it's configured, is ~500b smaller than the existing impl, or up to 4.6x faster. In its default setting it has slightly better code size than the existing impl, and is 2-2.7x faster.

Tested on all combinations of Armv7 arm/thumb2; Armv8 arm/thumb2/aarch64 on clang 14, gcc 11, and some more recent versions, plus Armv8 aarch64_be on gcc 7.5.

PR checklist

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• changelog provided
• development PR provided
• framework PR not required
• 3.6 PR not required because: non-functional change
• 2.28 PR not required because: non-functional change
• tests to follow via separate PR in mbedtls repo - see Mbed-TLS/mbedtls@a8e9a88

	multiblock setting	size delta c.f. development (aarch64)	perf delta c.f. development (aarch64)
clang	0	-84	0%
	1	-548	14%
	2	-344	64%
	3	-144	98%
	4	76	172%
	5	352	161%
	6	576	224%
gcc	0	-71	0%
	1	-519	57%
	2	-275	129%
	3	1	177%
	4	309	280%
	5	665	273%
	6	881	355%

[ronald-cron-arm]

Thanks for moving your PR to the new repo.