build: Allow archiver to be specified in the environment with HOST_AR.

R=rsc
CC=golang-dev
https://golang.org/cl/2515043

src/Make.clib

@@ -19,7 +19,7 @@ $(QUOTED_GOROOT)/lib/$(LIB): $(LIB)
 	cp $(LIB) "$(GOROOT)/lib/$(LIB)"
 
 $(LIB): $(OFILES)
-	ar rsc $(LIB) $(OFILES)
+	$(HOST_AR) rsc $(LIB) $(OFILES)
 
 CLEANFILES+=y.tab.[ch] y.output a.out $(LIB)
 

src/Make.inc

@@ -101,6 +101,7 @@ HOST_CC=quietgcc
 HOST_LD=quietgcc
 HOST_O=o
 HOST_YFLAGS=-d
+HOST_AR?=ar
 
 # These two variables can be overridden in the environment
 # to build with other flags.  They are like $CFLAGS and $LDFLAGS