tool/gocross: treat empty GOOS/GOARCH as native GOOS/GOARCH

Tracking down the side effect can otherwise be a pain, for example on
Darwin an empty GOOS resulted in CGO being implicitly disabled. The user
intended for `export GOOS=` to act like unset, and while this is a
misunderstanding, the main toolchain would treat it this way.

Fixes tailscale/corp#20059

Signed-off-by: James Tucker <james@tailscale.com>

tool/gocross/autoflags.go

@@ -4,6 +4,7 @@
 package main
 
 import (
+	"cmp"
 	"fmt"
 	"runtime"
 	"strings"
@@ -32,8 +33,8 @@ func autoflagsForTest(argv []string, env *Environment, goroot, nativeGOOS, nativ
 		subcommand = ""
 
 		cc          = "cc"
-		targetOS    = env.Get("GOOS", nativeGOOS)
-		targetArch  = env.Get("GOARCH", nativeGOARCH)
+		targetOS    = cmp.Or(env.Get("GOOS", ""), nativeGOOS)
+		targetArch  = cmp.Or(env.Get("GOARCH", ""), nativeGOARCH)
 		buildFlags  = []string{"-trimpath"}
 		cgoCflags   = []string{"-O3", "-std=gnu11"}
 		cgoLdflags  []string