This PR fixes broken links in all files describing libc usage modes.
Please let me know if there are any other places that need updating.

---------

Co-authored-by: shubhp@perlmutter <shubhp@perlmutter.com>

This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

This defines to LIBC_NAMESPACE with
`__attribute__((visibility("hidden")))` so all the symbols under it have
hidden visibility. This new macro should be used when declaring a new
namespace that will have internal functions/globals and LIBC_NAMESPACE
should be used as a means of accessing functions/globals declared within
LIBC_NAMESPACE_DECL.

It would be helpful in future code reviews to document a policy with
regards to
where and when Assembler sources are appropriate. That way when
reviewers point
out infractions, they can point to this written policy, which may help
contributors understand that it's not solely the personal preferences of
individual reviewers but instead rather a previously agreed upon rule by
maintainers.

Link: https://github.com/llvm/llvm-project/pull/87837
Link: https://github.com/llvm/llvm-project/pull/88157
Link:
https://discourse.llvm.org/t/hand-written-in-assembly-in-libc-setjmp-longjmp/73249/12

When supporting "overlay" vs "fullbuild" modes, "what ABI are you
using?" becomes a fundamental question to have concrete answers for.
Overlay mode MUST match the ABI of the system being overlayed onto;
fullbuild more flexible (the only system ABI relevant is the OS kernel).

When implementing llvm-libc we generally prefer the include-what-you use
style of avoiding transitive dependencies (since that makes refactoring
headers more painful, and slows down build times). So what header do you
include for any given type or function declaration? For any given
userspace program, the answer is straightforward. But for llvm-libc
which is trying to support multiple ABIs (at least one per
configuration), the answer is perhaps less clear.

This proposal seeks to add one layer of indirection relative to what's
being done today.

It then converts users of sigset_t and struct epoll_event and the epoll
implemenations over to this convention as an example.

Fixes #86546 and removes the macro `LIBC_HAS_BUILTIN`. This was
necessary to support older compilers that did not support
`__has_builtin`. All of the compilers we support already have this
builtin.
See: https://libc.llvm.org/compiler_support.html
All uses now use `__has_builtin` directly

cc @nickdesaulniers

Umbrella bug #83182

This reverts commit 6886a52d6dbefff77f33de12ff85d654e2557f81.

Most of the errors observed in postsubmit have been addressed. We can
fix-forward the remaining ones.

Link: https://lab.llvm.org/buildbot/#/changes/117129