* Added --tool flag that allows user to specify "gccgo" as the tool to build.
* Added builderEnv struct to abstract away setting up the build.
* Made envv and envvWindows methods of the builderEnv struct.
* Modified Builder.envv() to wrap envv and envvWindows
* Added internal builderEnv in Builder.
R=adg
CC=golang-dev
https://golang.org/cl/13240044
