cmd/go: add test case for cgo CC setting

Change-Id: Ied986053a64447c5eac6369f6c9b69ed3d3f94d9
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/949415
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/284782
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Roland Shoemaker <roland@golang.org>

src/cmd/go/testdata/script/cgo_path.txt

@@ -0,0 +1,35 @@
+[!cgo] skip
+
+env GOCACHE=$WORK/gocache  # Looking for compile flags, so need a clean cache.
+[!windows] env PATH=.:$PATH
+[!windows] chmod 0777 p/gcc p/clang
+[!windows] exists -exec p/gcc p/clang
+[windows] exists -exec p/gcc.bat p/clang.bat
+! exists p/bug.txt
+go build -x
+! exists p/bug.txt
+
+-- go.mod --
+module m
+
+-- m.go --
+package m
+
+import _ "m/p"
+
+-- p/p.go --
+package p
+
+// #define X 1
+import "C"
+
+-- p/gcc --
+#!/bin/sh
+echo ran gcc >bug.txt
+-- p/clang --
+#!/bin/sh
+echo ran clang >bug.txt
+-- p/gcc.bat --
+echo ran gcc >bug.txt
+-- p/clang.bat --
+echo ran clang >bug.txt