string: Replace outdated comments in strlen().

Copyright The GNU Toolchain Authors.

The comments on strlen() don't match what the actual code does. They
describe an older algorithm which is no longer in use. This change
replace the old comments with new ones describing the algorithm used.

I am a first time contributor, and I believe there is no need for
copyright assignment, since the file changed is not in the shared
source files list.

This patch only changes comments, but for safety I have run the tests in
my x64 ubuntu machine, with the following results:

Summary of test results:
   5051 PASS
     80 UNSUPPORTED
     16 XFAIL
      6 XPASS

Signed-off-by: Ricardo Bittencourt <bluepenguin@gmail.com>