internal/bytealg: add MTE-safe IndexByte implementation for ARM64 iOS

[strimziUser]

Add optimized ARM64 implementation of IndexByte and IndexByteString
for iOS that never reads before string start or beyond end,
making it MTE-safe.

Also extend the findnull fallback to iOS since it
may use ARM64 with restrictions similar to Plan 9.

Includes comprehensive tests for different SIMD code paths and alignments.

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Updates #59090

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