All functions in the API-MS-Win-Core-RtlSupport-L1-1-0 set are exports from both KERNEL32 and NTDLL, mostly from x64 builds only:

For most of these functions, the implementations in KERNEL32 version 6.1 and higher are just stubs which transfer the handling to wherever the schema redirects the API Set. The exceptions are:

Non-trivial implementations of all functions in this API Set are exported from NTDLL version 6.1 and higher.

Schema Redirection

The Windows 7 schema redirects this API Set to NTDLL. Thus: