[libc++] Support per-target __config_site in per-target runtime build

[libc++] Support per-target __config_site in per-target runtime build

When using the per-target runtime build, it may be desirable to have
different __config_site headers for each target where all targets cannot
share a single configuration.

The layout used for libc++ headers after this change is:

```
include/
  c++/
    v1/
      <libc++ headers except for __config_site>
  <target1>/
    c++/
      v1/
        __config_site
  <target2>/
    c++/
      v1/
        __config_site
  <other targets>
```

This is the most optimal layout since it avoids duplication, the only
headers that's per-target is __config_site, all other headers are
shared across targets. This also means that we no need two
-isystem flags: one for the target-agnostic headers and one for
the target specific headers.

Differential Revision: https://reviews.llvm.org/D89013