rtld: actually resolve memcpy plt

The call to memcpy() meant to cause plt resolution in _thr_rtld_init()
was getting optimized by the compiler. Tell the compiler not to use its
builtins in thr_rtld.c. We could avoid just the memcpy builtin but
disabling all will be more robust against future changes.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D45891