build: define getcallerpc in u.h (fix for Plan 9 build)
build: define getcallerpc in u.h (fix for Plan 9 build)

. By defining getcallerpc(x) as __builtin_return_address(0)
  here, it becomes possible to use the Plan 9 compatible form
  when compiling using GCC.  The alternative is to add conditional
  compilation based on the compiler identity in "cmd/8g/gsubr.c"
  to distinguish between the two cases.

R=golang-dev
CC=golang-dev, rsc
http://codereview.appspot.com/4800048

Committer: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old