Pass gcc builtin dir to just the linker

Bug: http://b/35402623

Pass the gcc builtin directory to CMake via linker flags instead of C
flags.  This in turn allows us to turn on
-Wunused-command-line-argument.

Turns out we also need an extra define for libfuzzer since the
CMakeLists.txt doesn't set a minimum version.

Test: python toolchain/llvm/android/build.py on Linux
Change-Id: I75e4bbbf69e947ec25fb5f8bc2d058291f71e790