cmd/go: document GOGCCFLAGS

Fixes #30123

Change-Id: I310b89a344a262bb758d39dfdd485ed2940cc6aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/166577
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/alldocs.go

@@ -1554,6 +1554,8 @@
 //
 // 	GOEXE
 // 		The executable file name suffix (".exe" on Windows, "" on other systems).
+// 	GOGCCFLAGS
+// 		A space-separated list of arguments supplied to the CC command.
 // 	GOHOSTARCH
 // 		The architecture (GOARCH) of the Go toolchain binaries.
 // 	GOHOSTOS

src/cmd/go/internal/help/helpdoc.go

@@ -587,6 +587,8 @@ Additional information available from 'go env' but not read from the environment
 
 	GOEXE
 		The executable file name suffix (".exe" on Windows, "" on other systems).
+	GOGCCFLAGS
+		A space-separated list of arguments supplied to the CC command.
 	GOHOSTARCH
 		The architecture (GOARCH) of the Go toolchain binaries.
 	GOHOSTOS