[PATCH, AArch64]: Optimize strlen

[Wilco Dijkstra]

Optimize the strlen implementation by using a page cross check and a fast check 
for nul bytes which reverts to separate loop when a non-ASCII char is encountered. 
Speedup on cortex-strings test is ~10%, long ASCII strings are processed ~60% 
faster, and on random tests it is ~80% better.

OK for commit?
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