Fix use of add_llvm_tool in projects that build no targets

Projects that set LLVM_TARGETS_TO_BUILD to an empty list
can't use add_llvm_tool (and probably other macros).

Here's the error that this change fixes:

list sub-command REMOVE_ITEM requires two or more arguments.

https://reviews.llvm.org/D70167



Signed-off-by: Kevin Petit <kevin.petit@arm.com>