Define strnlen when it is not in libc

[Joerg Sonnenberger via llvm-commits]

On Thu, Nov 24, 2016 at 02:06:55PM -0500, Chi-Hua Chen via llvm-commits wrote:
> Some older platforms, e.g. Mac 10.6.8, do not have strnlen. Define one
> in Support when necessary.

Personally, I'd prefer to just avoid it in first place. It is not really
much simpler than just using memchr.

Joerg