Knowing whether test failures are correlated with specific CPU models on
has proven useful on several issues. Log it for prior to testing so it
is always available.
internal/sysinfo provides the CPU model, but it is not available in the
bootstrap toolchain, so we can't access this unconditionally in
cmd/dist. Instead use a build-tagged file, as the final version of
cmd/dist will use the final toolchain.
The addition of new data to the beginning of cmd/dist output will break
x/build/cmd/coordinator's banner parsing, leaving extra lines in the log
output, though information will not be lost.
https://golang.org/cl/372538 fixes up the coordinator and should be
submitted and deployed before this CL is submitted.