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>

Triskite/willstone-goclone,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,npe9/noahevans-go9,scirelli/scirelli-go,scirelli/scirelli-go,d0f/go-zh,bryanxu/go-zh,bryanxu/go-zh,Triskite/willstone-goclone,npe9/noahevans-go9,Triskite/willstone-goclone,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,Triskite/willstone-goclone,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,d0f/go-zh,webfd/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,npe9/noahevans-go9,bryanxu/go-zh,bryanxu/go-zh,scirelli/scirelli-go,Triskite/willstone-goclone,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,d0f/go-zh,scirelli/scirelli-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,d0f/go-zh,mhennings/marcohennings-go,d0f/go-zh,webfd/go-zh,d0f/go-zh,webfd/go-zh,npe9/noahevans-go9,Triskite/willstone-goclone,scirelli/scirelli-go,sanjosh/sanjos100-tipc,Triskite/willstone-goclone,bryanxu/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,rflanagan/reginaldflanagan-project1,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,npe9/noahevans-go9,scirelli/scirelli-go