Reimplement strlen

The previous code neglected to use primitives which can find the end
of the string without having to branch on every character.

While here augment the somewhat misleading commentary -- strlen as
implemented here leaves performance on the table, especially so for
userspace. Every arch should get a dedicated variant instead.

In the meantime this commit lessens the problem.

Tested with glibc test suite.

Naive test just calling strlen in a loop on Haswell (ops/s):

$(perl -e "print 'A' x 3"):
before: 211198039
after: 338626619

$(perl -e "print 'A' x 100"):
before: 83151997
after: 98285919