The clang compiler on some terminals will issue colored error
messages, which can confuse tools like cgo. To avoid this we used to
set TERM=dumb for all programs started by the go tool. However, that
confuses the pprof tool, which doesn't know whether to support fancy
editing and colors itself.

Instead, change the go tool and the cgo tool to set TERM=dumb where it
matters--when invoking the C compiler--rather than in all cases.

Updates #26254

Change-Id: I95174f961ac269a50a83f5f9d268219043cba968
Reviewed-on: https://go-review.googlesource.com/122975
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>