libc: fix typo in __RENAME macro

Apparently __strncpy_real redirects to strcpy instead of strncpy for GCC
builds. This is bad, and it confused a project compiled by GCC that uses
strcpy.

I audited all of the FORTIFY-related __RENAMEs both manually and with a
script I hacked together; they all look correct aside from this one.

FWIW, with the cleaned up clang FORTIFY, lots of the `void foo()
__RENAME(foo)` will just become `void foo()` if this last thing I'm
working on gets through review.

Bug: 35329524
Test: Previously broken project now seems to work. Bullhead and Ryu
both build + boot.

Change-Id: Ib18ee3535ae31eb7e8ae846dc012f9b64cac80bf