All functions in the API-MS-Win-Core-String-L1-1-0 set are documented exports from KERNEL32:

The implementations in KERNEL32 version 6.1 and higher are just stubs which transfer the handling to wherever the schema redirects the API Set.

New Locations

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

Schema Redirection

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