c++: let plain -Wabi warn about future changes

c_common_post_options limits flag_abi_version and flag_abi_compat_version to
actual ABI version numbers, but let's not do that for warn_abi_version; we
might want to add a warning relative to a future ABI version that isn't
available in the current release, such backporting the PR120012 warning.

Also allow plain -Wabi to include such a warning without complaining that
it's useless.

Also warn about an unsupported -fabi-version argument.

gcc/c-family/ChangeLog:

	* c-opts.cc (c_common_post_options): Let plain -Wabi warn
	about changes in a future version.

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