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>

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