ltmain.sh: allow more flags at link-time

libtool defaults to filtering flags passed at link-time.

This brings the filtering in GCC's 'fork' of libtool into sync with
upstream libtool commit 22a7e547e9857fc94fe5bc7c921d9a4b49c09f8e.

In particular, this now allows some harmless diagnostic flags (especially
useful for things like -Werror=odr), more optimization flags, and some
Clang-specific options.

GCC's -flto documentation mentions:
> To use the link-time optimizer, -flto and optimization options should be
> specified at compile time and during the final link. It is recommended
> that you compile all the files participating in the same link with the
> same options and also specify those options at link time.

This allows compliance with that.

	* ltmain.sh (func_mode_link): Allow various flags through filter.