btx: rename .s files 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 are
assembled with Clang's integrated assembler; rename two files in
stand/i386/btx/lib to .S to use IAS as well.

The generated .text is identical (the entire .o files are not, as Clang
adds debug info).

PR:		205250, 233094
Discussed with:	imp
Sponsored by:	The FreeBSD Foundation

stand/i386/btx/lib/Makefile

@@ -4,7 +4,7 @@
 
 PROG=	crt0.o
 INTERNALPROG=
-SRCS=	btxcsu.S btxsys.s btxv86.s
+SRCS=	btxcsu.S btxsys.S btxv86.S
 CFLAGS+=-I${BOOTSRC}/i386/common
 LDFLAGS+=-Wl,-r