Switch libc default to clang.

* Default to clang when USE_CLANG_PLATFORM_BUILD is not set
  and the target has no clang bug.

BUG: 26102335
Change-Id: Ied6c9dc5593bfbadbb8d8b38e66ea237d649bae5
diff --git a/libc/Android.mk b/libc/Android.mk
index 04941f1..375b342 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -632,7 +632,7 @@
 endif
 
 ifeq ($(use_clang),)
-  use_clang := false
+  use_clang := true
 endif
 
 # Try to catch typical 32-bit assumptions that break with 64-bit pointers.