[libc++] Don't override LIBCXX_CXX_ABI_INCLUDE_PATHS for libcxxrt

When testing libc++ for our cross-compiled CheriBSD target we specify an
explicit LIBCXX_CXX_ABI_INCLUDE_PATHS for libcxxrt. The hardcoded path
/usr/include/c++/v1 was introduced in 61e89737
and overrides any value passed on the CMake command line. Fix this by using
it as a fallback rather than a fixed default value.

Reviewed By: #libc, ldionne
Differential Revision: https://reviews.llvm.org/D82095