pkg/runtime/Makefile:
. Adjusted so "goc2c.c" is built using the Plan 9 libraries.

pkg/runtime/goc2c.c:
. Added/subtracted #include headers to correspond to Plan 9
  toolkit.
. Changed fprintf(stderr,...)/exit() combinations to
  sysfatal() calls, adjusted the "%u" format to "%ud".
. Added exits(0) at the end of main().
. Made main() a void-returning function and removed the
  "return 0" at the end of it.

Tested on UBUNTU and Plan 9 only.

R=r, rsc
CC=golang-dev
https://golang.org/cl/4626093