Harden and adjust 'gcc/configure' parsing of '--enable-offload-targets'

Fix-up for commit d228ee80 "re PR
bootstrap/92314 (missing omp-device-properties', needed by
's-omp-device-properties-h')".

	gcc/
	* configure.ac <$enable_offload_targets>: Do parsing as done
	elsewhere.
	* configure: Regenerate.