Add support for using the secondary SDLLVM toolchain

With this patch, individual modules can choose to use the secondary SDLLVM
toolchain by setting LOCAL_SDCLANG_2 to true in their makefiles. Note that
LOCAL_SDCLANG and LOCAL_SDCLANG_2 should not be set to true at the same time.

Change-Id: I820230b1c449ab065e65648fbeaf47f347ced19e