[libc++] Split up ABI and platform configuration to their own headers (#90863)

This is a first step towards splitting up the <__config> header. The
<__config> header is large and rather disorganized at this point,
leading to confusion and subtle mistakes. For example, we never noticed
that the string layout used on arm64 was only enabled for the Clang
compiler, as the setting being in the compiler == clang block was
probably never intentional.

The danger of splitting up the <__config> header is to implicitly use
undefined macros that should have been defined prior to their usage,
however this can be remediated with -Wundef and we've started moving
towards -Wundef enforceable macros.