stand/i386: rename .s to .S to use Clang IAS

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.

PR: 233611
Reviewed by: tsoome
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18369