As part of the migration away from obsolete binutils we want to retire GNU as. Most assembly files used on amd64 have a .S extension and (via rules in share/mk/bsd.suffixes.mk) are assembled with Clang's Integrated Assembler (IAS). Rename files in stand/i386 to .S to use the integrated assembler.
Clang's IAS supports the defsym option (via -Wa,) but only with one dash, not two. As both -defsym and --defsym are accepted by GNU as, use the former.