runtime: handle locked mmap failure on Linux
runtime: handle locked mmap failure on Linux

Used to then die on a nil pointer situation. Most Linux standard setups are rather
restrictive regarding the default amount of lockable memory.

R=minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/6997049

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

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