x86-64 ABI requires arrays greater than 16 bytes to be aligned to

x86-64 ABI requires arrays greater than 16 bytes to be aligned to
16byte boundary.  However, GCC 16-byte aligns arrays of >=16 BITS,
not BYTES.

This diff improves bug detectability for code which has local arrays
of [16 .. 127] bits:  in those cases SSP will now detect even 1-byte
overflows.

OK kettenis@.  Tested in snaps for a week.