Disconnect -Wabi=<n> from -fabi-compat-version=<n>.

Disconnect -Wabi=<n> from -fabi-compat-version=<n>.

gcc/c-family/
	* c-common.h (abi_compat_version_crosses): New.
	(warn_abi_version): Declare.
	* c-common.c: Define it.
	* c-opts.c (c_common_post_options): Handle it.
	flag_abi_compat_version defaults to 8.
gcc/cp/
	* mangle.c (abi_warn_or_compat_version_crosses): New.
	Use it instead of abi_version_crosses.
	(mangle_decl): Deal with -fabi-compat-version and -Wabi separately.

From-SVN: r228017