Revert "amd64: implement strlen in assembly"

This reverts commit af366d353b84bdc4e730f0fc563853abc338271c.

Trips over '\xa4' byte and terminates early, as found in
lib/libc/gen/setdomainname_test:setdomainname_basic testcase

However, keep moving libkern/strlen.c out of conf/files.

Reported by: lwhsu