s390: Fix TARGET_SECONDARY_RELOAD for non-SYMBOL_REFs

RTX X need not necessarily be a SYMBOL_REF and may e.g. be an
UNSPEC_GOTENT for which SYMBOL_FLAG_NOTALIGN2_P fails.

gcc/ChangeLog:

	* config/s390/s390.cc (s390_secondary_reload): Guard
	SYMBOL_FLAG_NOTALIGN2_P.