Delete -x flag because tcc does not know about it

At some point I should delete other unused flags too.

sys/arch/amd64/conf/Makefile.amd64

@@ -77,7 +77,7 @@ CWARNFLAGS+=	-Wno-address-of-packed-member -Wno-constant-conversion \
 DEBUG?=		-g
 COPTIMIZE?=	-O2
 CFLAGS=		${DEBUG} ${CWARNFLAGS} ${CMACHFLAGS} ${COPTIMIZE} ${COPTS} ${PIPE}
-AFLAGS=		-D_LOCORE -x assembler-with-cpp ${CWARNFLAGS} ${CMACHFLAGS}
+AFLAGS=		-D_LOCORE ${CWARNFLAGS} ${CMACHFLAGS}
 LINKFLAGS=	-T ld.script -X --warn-common -nopie
 
 HOSTCC?=	${CC}