/go/src/runtime/cgo/cgo.go (in the tarball, installed into /usr/lib/ here) contains:
> #cgo CFLAGS: -Wall -Werror -fno-stack-protector
This seems to add -Werror to everything that is compiled with cgo.
-Werror is not recommended for releases and should always be disabled when encountered in build-logs, because there are numerous cases where this breaks without purpose.
See also: https://devmanual.gentoo.org/ebuild-writing/common-mistakes/index.html#-werror-compiler-flag-not-removed
*** Bug 887229 has been marked as a duplicate of this bug. ***