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