Reland Refactor WIDE_READ to allow finer control over high-performance function selection (#165613) (#170738)

[Previous commit had an incorrect default case when
FIND_FIRST_CHARACTER_WIDE_READ_IMPL was not specified in config.json.
This PR is identical to that one with one line fixed.]

As we implement more high-performance string-related functions, we have
found a need for better control over their selection than the big-hammer
LIBC_CONF_STRING_LENGTH_WIDE_READ. For example, I have a memchr
implementation coming, and unless I implement it in every variant, a
simple binary value doesn't work.

This PR makes gives finer-grained control over high-performance
functions than the generic LIBC_CONF_UNSAFE_WIDE_READ option. For any
function they like, the user can now select one of four implementations
at build time:

1. element, which reads byte-by-byte (or wchar by wchar)
2. wide, which reads by unsigned long
3. generic, which uses standard clang vector implemenations, if
available
4. arch, which uses an architecture-specific implemenation

(Reading the code carefully, you may note that a user can actually
specify any namespace they want, so we aren't technically limited to
those 4.)

We may also want to switch from command-line #defines as it is currently
done, to something more like
llvm-project/llvm/include/llvm/Config/llvm-config.h.cmake, and
complexity out of the command-line. But that's a future problem.