Make temp dir in cross-platform way

Since tempnam() seems to behave different on Linux and Windows.

autoload/go/debug.vim

@@ -399,8 +399,8 @@ function! go#debug#StartWith(...) abort
     " current dir. We pass --wd= so the binary is still run from the current
     " dir.
     let original_dir = getcwd()
-    let tmp = fnamemodify(tempname(), ':h')
     let pkgname = go#package#FromPath(bufname(''))
+    let tmp = go#util#tempdir('vim-go-debug-')
     exe 'lcd ' . tmp
 
     echohl SpecialKey | echomsg 'Starting GoDebug...' | echohl None

autoload/go/util.vim

@@ -362,7 +362,6 @@ function! go#util#archive()
     return expand("%:p:gs!\\!/!") . "\n" . strlen(l:buffer) . "\n" . l:buffer
 endfunction
 
-
 " Make a named temporary directory which starts with "prefix".
 "
 " Unfortunately Vim's tempname() is not portable enough across various systems;