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

stand/i386/kgzldr/Makefile

@@ -7,7 +7,7 @@ STRIP=
 BINMODE=${LIBMODE}
 BINDIR=	${LIBDIR}
 
-SRCS=	start.s boot.c subr_inflate.c lib.c crt.s sio.s
+SRCS=	start.S boot.c subr_inflate.c lib.c crt.S sio.S
 CFLAGS=	-Os
 CFLAGS+=-DKZIP
 NO_SHARED=
@@ -15,6 +15,6 @@ LDFLAGS+=-Wl,-r
 .PATH:	${SYSDIR}/kern
 
 BOOT_COMCONSOLE_PORT?=	0x3f8
-AFLAGS+=--defsym SIO_PRT=${BOOT_COMCONSOLE_PORT}
+ACFLAGS+=-Wa,-defsym,SIO_PRT=${BOOT_COMCONSOLE_PORT}
 
 .include <bsd.prog.mk>

stand/i386/mbr/Makefile

@@ -3,14 +3,14 @@
 PROG=	mbr
 STRIP=
 BINMODE=${NOBINMODE}
-SRCS=	${PROG}.s
+SRCS=	${PROG}.S
 
 # MBR flags: 0x80 -- try packet interface (also known as EDD or LBA)
 BOOT_MBR_FLAGS?=	0x80
 
 ORG=	0x600
 
-AFLAGS+=--defsym FLAGS=${BOOT_MBR_FLAGS}
+ACFLAGS+=-Wa,-defsym,FLAGS=${BOOT_MBR_FLAGS}
 LDFLAGS+=${LDFLAGS_BIN}
 
 .include <bsd.prog.mk>

stand/i386/pmbr/Makefile

@@ -3,11 +3,13 @@
 PROG=	pmbr
 STRIP=
 BINMODE=${NOBINMODE}
-SRCS=	${PROG}.s
+SRCS=	${PROG}.S
+
+BOOT_MBR_FLAGS?=	0
 
 ORG=	0x600
 
-AFLAGS+=--defsym FLAGS=${BOOT_MBR_FLAGS}
+ACFLAGS+=-Wa,-defsym,FLAGS=${BOOT_MBR_FLAGS}
 LDFLAGS+=${LDFLAGS_BIN}
 
 .include <bsd.prog.mk>