[PATCH] CrossCompile: Escape LLVM_TARGET variable properly

Without this patch, if the user provides a list of targets such as
-DLLVM_TARGETS_TO_BUILD=a,b the NATIVE cross compilation folder will
only contain target a, due to the way cmake parses variables.

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>