It seems like C++'s lack of an ABI would bite you way before any issues with libc++. (Or is there one now? I'm getting old.)
This document was developed jointly by an informal
industry coalition consisting of (in alphabetical order)
CodeSourcery, Compaq, EDG, HP, IBM, Intel, Red Hat, and
SGI. Additional contributions were provided by a variety of
individuals.
...
In general, this document is written as a generic
specification, to be usable by C++ implementations on a
variety of architectures.
As most OSs tend to be written mostly in C, it is natural that C ABI == OS ABI.
In the few cases where the OS was written in other languages, like the mainframe systems, the early 80's OS done in Pascal dialects (Mac OS), Modula-2 (Lillith), Assembly (MS-DOS/CP-M) and many others to list here, there was no C ABI to speak of.