GCC_CET_HOST_FLAGS: Check if host supports multi-byte NOPs

Sync with binutils for PR binutils/27397.  Check if host supports
multi-byte NOPs before enabling CET on host.

config/

	PR bootstrap/99703
	* cet.m4 (GCC_CET_HOST_FLAGS): Check if host supports multi-byte
	NOPs.

libiberty/

	PR bootstrap/99703
	* configure: Regenerated.