[libc++] Add a 'is-lockfree-runtime-function' lit feature
On macOS 10.14 /usr/lib/system/libcompiler_rt.dylib contains all the
__atomic_load*, etc. functions but does not include the __atomic_is_lock_free
function. The lack of this function causes the non-lockfree-atomics feature
to be set to false even though large atomic operations are actually
supported, it's just the is_lock_free() function that is missing.
This is required so that the !non-lockfree-atomics feature can be used
to XFAIL tests that require runtime library support (D88818).
Reviewed By: #libc, ldionne
Differential Revision: https://reviews.llvm.org/D91911