Enable CET in cross compiler if possible

Don't perform CET run-time check for host when cross compiling.  Instead,
enable CET in cross compiler if possible so that it will run on both CET
and non-CET hosts.

config/

	PR bootstrap/94998
	* cet.m4 (GCC_CET_HOST_FLAGS): Enable CET in cross compiler if
	possible.

libiberty/

	PR bootstrap/94998
	* configure: Regenerated.

lto-plugin/

	PR bootstrap/94998
	* configure: Regenerated.