runtime: delete dead code called from C.

runtime: delete dead code called from C.

printf, vprintf, snprintf, gc_m_ptr, gc_g_ptr, gc_itab_ptr, gc_unixnanotime.

These were called from C.
There is no more C.

Now that vprintf is gone, delete roundup, which is unsafe (see CL 2814).

Change-Id: If8a7b727d497ffa13165c0d3a1ed62abc18f008c
Reviewed-on: https://go-review.googlesource.com/2824
Reviewed-by: Austin Clements <austin@google.com>