gc: use HEADER_IO macro from gopack
gc: use HEADER_IO macro from gopack

Use HEADER_IO macro from gopack to read archive header
The HEADER_IO macro portably reads archive headers. The
current arsize code fails in the case of archive headers produced
on plan 9 6c and read on other systems (it's not portable).
Modify lex.c to use the portable macro
Build tested (including tests) on OSX.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5323072

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

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