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
https://golang.org/cl/5323072