This change adds a 32-bit builder for gccgo and fixes some specific
configuration problems with the docker image/coordinator to speed up builds.
The go builder running on the linux-x86-gccgo image has been modified to run
`make -j16` when building to reduce build times from ~60 minutes to ~10 minutes
and the coordinator has been modified to run the command `make check-go -j16`
to reduce testsuite run times from ~30 minutes to ~10 minutes.
LGTM=bradfitz
R=bradfitz
CC=adg, golang-codereviews
https://golang.org/cl/151530043
