[LLD][MinGW] Add --[no-]guard-cf and --[no-]guard-longjmp

These will be LLD-specific options to support Control Flow Guard for the
MinGW target. They are disabled by default, but enabling `--guard-cf`
will also enable `--guard-longjmp` unless `--no-guard-longjmp` is also
specified. These options maps to `-guard:cf,[no]longjmp`.

Note that these features require the `_load_config_used` symbol to
contain the load config directory and be filled with the required
symbols. While current versions of mingw-w64 do not supply this symbol,
the user can provide their own version of it.

Reviewed By: MaskRay, rnk

Differential Revision: https://reviews.llvm.org/D132808