internal/goroot: replace HasPrefix+TrimPrefix with CutPrefix

Change-Id: I97305df6babbede57bb0c3b48c89c96cb74307f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/668456
Auto-Submit: Sean Liao <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Sean Liao <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: cuishuang

src/internal/goroot/gc.go

@@ -77,8 +77,8 @@ func (gd *gccgoDirs) init() {
 	const prefix = "libraries: ="
 	var dirs []string
 	for _, dirEntry := range dirsEntries {
-		if strings.HasPrefix(dirEntry, prefix) {
-			dirs = filepath.SplitList(strings.TrimPrefix(dirEntry, prefix))
+		if after, ok := strings.CutPrefix(dirEntry, prefix); ok {
+			dirs = filepath.SplitList(after)
 			break
 		}
 	}