Reland: "[libc] Templatize str{,n}cmp"

This will be used to implement the case insensitive str{,n}casecmp

This was initially reverted because it broke tests on arm platforms.
Unfortunately, it didn't break on my arm machine, but I suspect the
problem was the old comparator returned char and not int.

Differential Revision: https://reviews.llvm.org/D141235