IBM Z: Cover weak symbols with -munaligned-symbols

With the recently introduced -munaligned-symbols option byte-sized
variables which are resolved externally are considered to be
potentially misaligned.
However, this should rather also be applied to symbols which resolve
locally if they are weak. Done with this patch.

gcc/ChangeLog:

	* config/s390/s390.cc (s390_encode_section_info): Replace
	SYMBOL_REF_LOCAL_P with decl_binds_to_current_def_p.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/unaligned-2.c: New test.