build/clang_darwin: Fix previous change to CheckArches to not cause problem when
command produces error output on stdout instead of stderr (and suppress the
stderr output while at it).

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@133019 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/make/platform/clang_darwin.mk b/make/platform/clang_darwin.mk
index 0d6dbcb..99baacf 100644
--- a/make/platform/clang_darwin.mk
+++ b/make/platform/clang_darwin.mk
@@ -12,7 +12,8 @@
   $(shell \
     result=""; \
     for arch in $(1); do \
-      if $(CC) -arch $$arch -c -x c /dev/null -o /dev/null; then \
+      if $(CC) -arch $$arch -c -x c /dev/null \
+	  -o /dev/null > /dev/null 2> /dev/null; then \
         result="$$result$$arch "; \
       fi; \
     done; \