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This enables goinstall to handle .go and .c files (for cgo)
which are named after the following patterns:

    name_$(GOOS).*
    name_$(GOARCH).*
    name_$(GOOS)_$(GOARCH).*

Files with those names are only included if the $(GOOS) and
$(GOARCH) match the current system.

R=rsc
CC=golang-dev
https://golang.org/cl/4172055