6l: correct offset for __nl_symbol_ptr in Mach-O.
6l: correct offset for __nl_symbol_ptr in Mach-O.

Fixes malformed object message from nm etc.
Fixes issue 1180.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/2390042

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

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