cmd/link/internal/ld: handle "\r" in MinGW "--print-prog-name" output

Fix the "gcc --print-prog-name" output parser to handle "\r\n", not only
"\n". The MinGW compiler on Windows uses "\r\n" as line endings, causing
the existing parser to create paths like
".../x86_64-w64-mingw32/bin/ar.exe\r", which is not correct. By trimming
the "\r\n" cutset, both types of line endings are handled correctly.

Fixes golang#68121

Change-Id: I04b8bf9b6a5b29a1e59a6aa07fa4faa4c5bdeee6
Reviewed-on: https://go-review.googlesource.com/c/go/+/593916
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
Commit-Queue: Ian Lance Taylor <[email protected]>

Author: dagood

src/cmd/link/internal/ld/lib.go

@@ -2917,6 +2917,6 @@ func (ctxt *Link) findExtLinkTool(toolname string) string {
 	if err != nil {
 		Exitf("%s: finding %s failed: %v\n%s", os.Args[0], toolname, err, out)
 	}
-	cmdpath := strings.TrimSuffix(string(out), "\n")
+	cmdpath := strings.TrimRight(string(out), "\r\n")
 	return cmdpath
 }