compilers: cpp: improve libc++ vs libstdc++ detection (again)

The previous approach wasn't great because you couldn't control what the
detected C++ stdlib impl was. We just had a preference list we tweaked the
searched order for per OS. That doesn't work great for e.g. Gentoo with libc++
or Gentoo Prefix on macOS where we might be using libstdc++ even though the host
is libc++.

Jonathan Wakely, the libstdc++ maintainer, gave a helpful answer on how
to best detect libc++ vs libstdc++ via macros on SO [0]. Implement it.

TL;DR: Use <version> from C++20 if we can, use <ciso646> otherwise. Check for
_LIBCPP_VERSION as libstdc++ doesn't always define a macro.

[0] https://stackoverflow.com/a/31658120

Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Eli Schwartz <eschwartz93@gmail.com>

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