Use llvm_canonicalize_cmake_booleans for LLVM_LIBXML2_ENABLED [lld]

r291284 added a nice mechanism to consistently pass CMake on/off toggles to
lit. This change uses it for LLVM_LIBXML2_ENABLED too (which was added around
the same time and doesn't use the new system yet).

No intended behavior change.

Differential Revision: https://reviews.llvm.org/D56912

llvm-svn: 351614