Remove warnings about mismatched types with builtins for stubs.

Looks like new versions of the toolchain have an error about mismatching
the definitions used in the stubs with the builtin definitions. Since we
don't care about this when building stubs, ignore it by disabling the
builtin definitions.

Bug: N/A
Test: Build platform with ToT LLVM compiler.
Change-Id: I4e449c5fb96d0d5442fd57a7fea02ba69463324b