cmd/go: clarify promotion of C warnings to errors

CL 23005 (back in 2016!) added logic to promote C compiler warnings to
errors when running on the Go builders.

CL 437298 kept the logic to promote warnings to errors on the
builders, but dropped the explanatory message, I believe
unintentionally. Indeed, now there isn't even a comment in the code
explaining what's going on.

This CL adds back an explanatory message to the printed output, which
also serves as a explanation in the code as to why we're checking
$GO_BUILDER_NAME.

Change-Id: I769c55d213f96f73d20a41ab926fb91e71a5a22c
Reviewed-on: https://go-review.googlesource.com/c/go/+/522775
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

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