use rustc -Vv in the run-make test

Author: japaric

src/test/run-make-fulldeps/emit-stack-sizes/Makefile

@@ -7,14 +7,14 @@ all:
 else
 # check that the .stack_sizes section is generated
 # this test requires LLVM >= 6.0.0
-vers = $(shell llvm-ar -version)
-ifneq (,$(findstring version 3,$(vers)))
+vers = $(shell $(RUSTC) -Vv)
+ifneq (,$(findstring LLVM version: 3,$(vers)))
 all:
 	exit 0
-else ifneq (,$(findstring version 4,$(vers)))
+else ifneq (,$(findstring LLVM version: 4,$(vers)))
 all:
 	exit 0
-else ifneq (,$(findstring version 5,$(vers)))
+else ifneq (,$(findstring LLVM version: 5,$(vers)))
 all:
 	exit 0
 else