This change adds solaris to the list of supported operating
systems and allows cmd/dist to be built on Solaris.
This CL has to come first because we want the tools to ignore
solaris-specific files until the whole port is integrated.
R=golang-codereviews, jsing, rsc, minux.ma
CC=golang-codereviews
https://golang.org/cl/35900045
Before, an empty TMPDIR would lead to:
cannot create <nil>/go.o: No such file or directory
R=golang-dev, iant, dave, bradfitz
CC=golang-dev
https://golang.org/cl/8355045
