Add conditional compilation to add support for AArch64 without vector
registers and/or hardware FPUs by using the generic implementation.

**Context:**

A few functions were hard-coded to use vector registers/hardware FPUs.
This meant that libc would not compile on architectures that did not
support these features. This fix falls back on the generic
implementation if a feature is not supported.

Fixed imports for all files *within* `libc/src/string/memory_utils`.
Note: This doesn't include **all** files that need to be fixed.

Fixes #86579

The patch primarily cleans up some incorrect includes. The `LIBC_INLINE`
macro is defined in `attributes.h`, not `config.h`. There appears to be
no need to change the CMake and Bazel build files.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.

Codify that we use lower_case for
readability-identifier-naming.ConstexprFunctionCase and then fix the 11
violations (rather than codify UPPER_CASE and have to fix the 170 violations).

Towards the goal of getting `ninja libc-lint` back to green, fix the numerous
instances of:

    warning: header guard does not follow preferred style [llvm-header-guard]

This is because many of our header guards start with `__LLVM` rather than
`LLVM`.

To filter just these warnings:

    $ ninja -k2000 libc-lint 2>&1 | grep llvm-header-guard

To automatically apply fixits:

    $ find libc/src libc/include libc/test -name \*.h | \
        xargs -n1 -I {} clang-tidy {} -p build/compile_commands.json \
        -checks='-*,llvm-header-guard' --fix --quiet

Some manual cleanup is still necessary as headers that were missing header
guards outright will have them inserted before the license block (we prefer
them after).

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

This patch mostly renames files so it better reflects the function they declare.

Reviewed By: michaelrj

Differential Revision: https://reviews.llvm.org/D155607