clang: Fix x86-no-gather-no-scatter.cpp on macOS after 993bdb04

On macOS, files are usually below `/Users/...` and clang-cl treats
that as the `/U` flag followed by something instead of as a path.
Put `--` in front of `%s` to make it treat it as a patch, like in
all other tests.

The failure without this change:

    x86-no-gather-no-scatter.cpp:4:14: error: NOGATHER: expected string not found in input
    // NOGATHER: "-target-feature" "+prefer-no-gather"
                 ^
    <stdin>:5:44: note: possible intended match here
    clang: warning: 'x86-no-gather-no-scatter.cpp' treated as the '/U' option [-Wslash-u-filename]
                                               ^