x/build: add racebuild utility
x/build: add racebuild utility

Racebuild builds race runtime (syso files) on all supported OSes using gomote.
For now only freebsd is supported.
Usage:
	$ racebuild -rev <llvm_revision> -goroot <path_to_go_repo>
The rest should happen automatically (provided you have gomote access).

Update golang/go#8653

Change-Id: Ie9df99860616084471dc92b8dbf369020fbb8f72
Reviewed-on: https://go-review.googlesource.com/16823
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

proppy/build,golang/build,golang/build,golang/build,proppy/build,golang/build,proppy/build,proppy/build