tools/pkg-config: pass arguments at the end

tools/pkg-config: pass arguments at the end

Go for openwrt passes pkg-config arguments in the format of

        pkg-config --cflags -- pkg-name

which in turn will be passed down to the real pkg-config as something
like

        pkg-config.real --cflags -- pkg-name --define...

and causes the real pkg-config implementation to missinterpret the given
argument list.

This also helps to fix https://github.com/golang/go/issues/27940

Signed-off-by: Arthur Skowronek <arthur.skowronek@tuta.io>