IBM Z: Fix -munaligned-symbols

With this fix we make sure that only symbols with a natural alignment
smaller than 2 are considered misaligned with
-munaligned-symbols. Background is that -munaligned-symbols is only
supposed to affect symbols whose natural alignment wouldn't be enough
to fulfill our ABI requirement of having all symbols at even
addresses. Because only these are the cases where we differ from other
architectures.

gcc/ChangeLog:

	* config/s390/s390.cc (s390_encode_section_info): Adjust the check
	for misaligned symbols.
	* config/s390/s390.opt: Improve documentation.

gcc/testsuite/ChangeLog:

	* gcc.target/s390/aligned-1.c: Add weak and void variables
	incorporating the cases from unaligned-2.c.
	* gcc.target/s390/unaligned-1.c: Likewise.
	* gcc.target/s390/unaligned-2.c: Removed.