Change USE_CLANG_PLATFORM_BUILD default to true.

When USE_CLANG_PLATFORM_BUILD is not set, default will be clang/llvm.
USE_CLANG_PLATFORM_BUILD=false can be used to select gcc as default.

Bug: 23163853
Bug: 26102335
Change-Id: I434176732fa4a382be9d8d8642a1c705b023cf84
diff --git a/core/binary.mk b/core/binary.mk
index 445b9d5..ebb40c8 100644
--- a/core/binary.mk
+++ b/core/binary.mk
@@ -217,11 +217,13 @@
         my_clang := true
     endif
     endif
-# Add option to make clang the default for device build
-else ifeq ($(USE_CLANG_PLATFORM_BUILD),true)
+# Add option to make gcc the default for device build
+else ifeq ($(USE_CLANG_PLATFORM_BUILD),false)
     ifeq ($(my_clang),)
-        my_clang := true
+        my_clang := false
     endif
+else ifeq ($(my_clang),)
+    my_clang := true
 endif
 
 my_cpp_std_version := -std=gnu++14