The C startup code calls exit(), which in turn called
__cxa_thread_call_dtors that called fprintf(), so stdio is
linked regardless of our effort.
MFC after: 2 weeks
Sidenote: This was introduced in rS121010 and I think the goal
was to reduce the size of statically linked binary of /bin/echo.
In rS308432 a call to err() was introduced, but rewriting it
as errexit() would not remove err.c from the statically linked
The space saving was negligible (7944 -> 7528 for FreeBSD/amd64
dynamically linked, or 766656 -> 766416 statically linked).