[TODO] Standardize presence/absence of libc++ headers

[L1ghtmann]

Currently provided by the Linux toolchains and some SDKS. Need to decide if we're going to follow Xcode, which I believe provides the headers in the SDKs, or not.

If keeping headers here, need to re-add to:
-12.4
-13.7
-14.5
and remove from Linux toolchains