Assemble .s to .o using cc, not as

As of commit fd71da37d4 we no longer have an `as` in the default
toolchain.  Although we do not make use of this rule in the base system
some ports or downstream projects might.  Use `cc -x assembler` instead
of as.

Reviewed by:	arichardson
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D35859

share/mk/bsd.compat.mk

@@ -37,7 +37,6 @@ LIB32_MACHINE=	i386
 LIB32_MACHINE_ARCH=	i386
 LIB32WMAKEENV=	MACHINE_CPU="i686 mmx sse sse2"
 LIB32WMAKEFLAGS=	\
-		AS="${XAS} --32" \
 		LD="${XLD} -m elf_i386_fbsd"
 
 .elif ${COMPAT_ARCH} == "powerpc64"

share/mk/bsd.lib.mk

@@ -192,7 +192,7 @@ PO_FLAG=-pg
 	${CTFCONVERT_CMD}
 
 .s.po .s.pico .s.nossppico .s.pieo:
-	${AS} ${AFLAGS} -o ${.TARGET} ${.IMPSRC}
+	${CC:N${CCACHE_BIN}} -x assembler ${ACFLAGS} -c ${.IMPSRC} -o ${.TARGET}
 	${CTFCONVERT_CMD}
 
 .asm.po:

share/mk/bsd.suffixes.mk

@@ -55,7 +55,7 @@
 	${CTFCONVERT_CMD}
 
 .s.o:
-	${AS} ${AFLAGS} -o ${.TARGET} ${.IMPSRC}
+	${CC:N${CCACHE_BIN}} -x assembler ${ACFLAGS} -c ${.IMPSRC} -o ${.TARGET}
 	${CTFCONVERT_CMD}
 
 # XXX not -j safe