Always prefer double dashes for GNU LD long options.

I discovered this while working on llvm/lld and realized export-dynamic
only supported --. Although upstream will eventually grow to support
both - and --, switch this in our build system, because GNU ld supports
both modes, and because there's some hope lld will become the default linker
for FreeBSD in the future.

Discussed with:		emaste, rdivacky

sys/conf/kern.pre.mk

@@ -174,7 +174,7 @@ SYSTEM_OBJS= locore.o ${MDOBJS} ${OBJS}
 SYSTEM_OBJS+= ${SYSTEM_CFILES:.c=.o}
 SYSTEM_OBJS+= hack.So
 SYSTEM_LD= @${LD} -Bdynamic -T ${LDSCRIPT} ${_LDFLAGS} --no-warn-mismatch \
-	-warn-common -export-dynamic -dynamic-linker /red/herring \
+	--warn-common --export-dynamic --dynamic-linker /red/herring \
 	-o ${.TARGET} -X ${SYSTEM_OBJS} vers.o
 SYSTEM_LD_TAIL= @${OBJCOPY} --strip-symbol gcc2_compiled. ${.TARGET} ; \
 	${SIZE} ${.TARGET} ; chmod 755 ${.TARGET}