lib/libc/aarch64/string: add optimized strpbrk & strsep implementations

[getz]

Summary

These are direct copies from the amd64 string functions using the
optimized strcspn from D46398

Test Plan

No regressions noticed

[fuz]

Ok.

lib/libc/aarch64/string/strsep.c

36

[getz]

• amd64 -> aarch64 :)

[andrew]

Could this be machine independent?

[fuz]

@andrew If we don't have an assembly implementation for strcspn, it's generally faster to implement this function directly.

[fuz]

exp-run says it's fine.