MFC r213326: libc: Remove the i386 assembler version of strlen(3).

MFC r213326: libc: Remove the i386 assembler version of strlen(3).

On anything modern, the C version, which processes a word at a time, is much
faster. The Intel optimization manual explicitly warns against using REP
prefixes with SCAS or CMPS, which is exactly what the assembler version
does.

A simple test on a Phenom II showed the C version, compiled with -O2, to be
about twice as fast determining the length of 100000 strings between 0 and
255 bytes long.

git-svn-id: svn://svn.freebsd.org/base/stable/8@214086 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f