auto merge of #7200 : yichoi/rust/fix_je_mac_cross, r=brson

while cross-compiling, ar in cross toolchains are required. 
linux is not sensitive so could not see errors.

src/rt/jemalloc/Makefile.in

@@ -9,6 +9,7 @@ vpath % .
 SHELL := /bin/sh
 
 CC := @CC@
+AR := @AR@
 
 # Configuration parameters.
 DESTDIR =

src/rt/jemalloc/configure

@@ -4453,7 +4453,7 @@ PIC_CFLAGS='-fPIC -DPIC'
 CTARGET='-o $@'
 LDTARGET='-o $@'
 EXTRA_LDFLAGS=
-MKLIB='ar crus $@'
+MKLIB='$(AR) crus $@'
 CC_MM=1
 
 default_munmap="1"