runtime/cgo: remove stdlib.h warning workaround on darwin
CL 205457 added the flag -Wno-nullability-completeness to work
around a user-reported build breakage on macOS Catalina.
However, according to
https://golang.org/issue/35247#issuecomment-
589115489 the root cause
of the breakage may be a toolchain misconfiguration on the host
(perhaps compiling the XCode stdlib using a Homebrew build of the
"clang" compiler?).
Adding an obscure warning flag to enable building stdlib.h with an
otherwise-broken toolchain seems clearly inappropriate to me.
If need be we can instead provide guidance to users on how to unbreak
their toolchain.
Updates #35247
Fixes #49913
Change-Id: I84def34e101bed7911d8d78a991a29095b8791fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/368634
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>