add -nobounds (similar to -gcflags=-B)

[dgryski]

No description provided.

[deadprogram]

Thanks for the addition @dgryski and to @eliasnaur for quick review. Now merging.

[aykevl]

Hmm, I wonder whether it would perhaps be cleaner to implement -gcflags=-B directly? It can be parsed in a similar way to how we parse -ldflags already.